Black magnetic acicular composite particles for magnetic recording medium and magnetic recording medium using the same

ABSTRACT

A magnetic recording medium comprises:
         a non-magnetic base film; and   a magnetic recording layer comprising a binder resin and black magnetic acicular composite particles having an average particle diameter of 0.051 to 0.35 μm, comprising   magnetic acicular cobalt-coated iron oxide particles or magnetic acicular metal particles containing iron as a main component,   a coating formed on surface of the magnetic acicular particles, comprising at least one organosilicon compound selected from the group consisting of:   (1) organosilane compounds obtainable from alkoxysilane compounds, and   (2) polysiloxanes or modified polysiloxanes, and
           a carbon black coat formed on the coating layer comprising said organosilicon compound, in an amount of from more than 10 to 40 parts by weight based on 100 parts by weight of said magnetic acicular particles.   
               

     Such a magnetic recording medium exhibits not only a smooth surface, a smaller light transmittance and a lower surface electrical resistivity value, but also a small friction coefficient and an excellent running durability.

BACKGROUND OF THE INVENTION

The present invention relates to magnetic acicular composite particles for magnetic recording medium, and a magnetic recording medium using the black magnetic acicular composite particles. More particularly, the present invention relates to magnetic acicular composite particles exhibiting not only an excellent dispersibility in vehicle due to a less amount of carbon black desorbed or fallen-off from the surfaces thereof, but also a more excellent blackness, a lower volume resistivity value and a well-controlled myristic acid-adsorption; and a magnetic recording medium exhibiting not only a smooth surface, a smaller light transmittance and a lower surface electrical resistivity value, but also a small friction coefficient and an excellent running durability.

With a development of miniaturized, lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly desired to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level.

Especially, video tapes have recently been desired more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.

In order to enhance output characteristics of magnetic recording media, especially an S/N ratio thereof with respect to signals having a short wavelength, there have been demanded fineness of magnetic particles, reduction in thickness of a magnetic recording layer, high dispersibility of magnetic particles and surface smoothness of a magnetic coating film.

On the other hand, at the present time, the end position of a magnetic recording medium such as magnetic tapes has been detected by sensing a high light transmittance portion of the magnetic recording medium by means of a video deck. In the case where the particle size of magnetic particles dispersed in the magnetic recording layer become finer and the thickness of the magnetic recording medium is reduced in order to meet the requirement for high performance of the magnetic recording medium as described hereinbefore, the magnetic recording medium shows a high light transmittance as a whole, so that it has been difficult to detect the end position thereof by means of the video deck. In order to solve this problem, carbon black fine particles have been added to the magnetic recording layer in an amount of usually about 6 to 12 parts by weight based on 100 parts by weight of the magnetic particles, thereby reducing the light transmittance of the magnetic recording medium. For this reason, in current videotapes, it is essential to add carbon black fine particles, etc., to the magnetic recording layer thereof.

However, when a large amount of such non-magnetic carbon black fine particles are added to the magnetic recording layer, the magnetic recording medium suffers from deterioration in signal recording property, thereby hindering high-density recording thereon, and the reduction in thickness of the magnetic recording layer becomes incapable. Further, due to the fact that the carbon black fine particles have an average particle size as fine as 0.002 to 0.05 μm and a large BET specific surface area value, and are deteriorated in solvent-wettability, it has been difficult to disperse these carbon black fine particles in vehicle, thereby failing to obtain a magnetic recording medium having a smooth surface.

Further, in the case where the magnetic recording medium has a high surface electrical resistivity, the electrostatic charge amount thereof is increased, so that cut chips or dusts tend to adhere onto the surface of the magnetic recording medium upon the production or use thereof, thereby causing such a problem that the dropout frequently occurs. Therefore, in order to reduce not only the light transmittance of the magnetic recording medium but also the surface electrical resistivity thereof, especially below about 10⁸ Ω/cm², the carbon black fine particles have been conventionally added to the magnetic recording layer of the magnetic recording medium.

However, as described above, in the case where the amount of such carbon black fine particles or the like which do not contribute to magnetic properties of the magnetic recording layer, is increased, there are caused such problems that the magnetic recording medium suffers from deterioration in signal recording property, the reduction in thickness of the magnetic recording layer becomes incapable, and further the surface smoothness of the magnetic recording layer is deteriorated.

Also, since the carbon black fine particles are bulky particles having a bulk density as low as about 0.1 g/cm³, the handling property and workability thereof are deteriorated. In addition, it has been pointed out that the use of such carbon black fine particles causes problems concerning safety and hygiene such as carcinogenesis.

Magnetic recording media have been continuously required to enhance performances thereof. In particular, it has been strongly required to improve physical properties of these magnetic recording media such as running property in addition to the above-described high-density recording property.

The running property of magnetic recording media (tapes) can be ensured by adding a fatty acid such as myristic acid or stearic acid (hereinafter referred to merely as “myristic acid”) in an amount of usually about 0.5 to 5% by weight based on the weight of magnetic particles, into a magnetic recording layer generally formed as an upper layer of the respective magnetic recording medium, and then allowing the myristic acid to be gradually oozed out on the surface of the magnetic recording layer so as to render the surface slidable.

When the amount of the myristic acid oozed out on the surface of the magnetic recording layer is too small, it is not possible to ensure a good running property of the magnetic recording media. On the contrary, when a large amount of the myristic acid is added to the magnetic recording layer so as to allow a large amount of myristic acid to be subsequently oozed out on the surface thereof, the myristic acid is preferentially adsorbed onto the surface of each magnetic particle dispersed in the magnetic recording layer, thereby inhibiting the magnetic particles from being contacted with or adsorbed into resins. As a result, it is difficult to disperse the magnetic particles in vehicle. Also, the increase in amount of the myristic acid as a non-magnetic component causes deterioration in magnetic properties of the magnetic recording media. Further, since the myristic acid acts as a plasticizer, there arise problems such as deterioration in mechanical strength of the magnetic recording media.

Recently, with further reduction in thickness of the magnetic recording layer, the absolute amount of myristic acid added to the magnetic recording layer is decreased. In addition, since the particle size of the magnetic particles have become much finer in order to meet the requirement of high-density recording, the BET specific surface area thereof is increased, so that a large amount of myristic acid is absorbed onto the surfaces of the magnetic particles. Under these conditions, it is more and more difficult to properly adjust the amount of the myristic acid oozed out on the surface of the magnetic recording layer by the amount of the myristic acid added into the magnetic recording layer, thereby ensuring a good running property of the magnetic recording layer.

Accordingly, it has been strongly required to properly adjust the amount of myristic acid oozed out on the surface of the magnetic recording layer and ensure a good running property of the magnetic recording media.

In order to improve electrical resistance and running durability of the magnetic recording media, there have been proposed such magnetic particles on the surfaces of which carbon and/or graphite are deposited or adhered in an amount of 0.2 to 10.0% by weight based on the weight of the magnetic particles (corresponding to 0.2 to 11.11 parts by weight based on 100 parts by weight of the magnetic particles) by various methods such as gas-phase growth, e.g., chemical vapor deposition (CVD) or physical vapor deposition (PVD), reduction of organic compounds, and thermal decomposition or incomplete combustion of hydrocarbons (Japanese Patent Application Laid-Open (KOKAI) No. 10-269558(1998)).

Also, hitherto, with the reduction in thicknesses of magnetic recording layer and non-magnetic base film of magnetic recording media, it has been variously attempted to impart good surface smoothness and large stiffness thereto by improving a substrate on which the magnetic recording layer is formed. For instance, there has been proposed a non-magnetic substrate composed of a non-magnetic base film and at least one undercoat layer formed on the non-magnetic base film. The undercoat layer is composed of a binder and non-magnetic particles dispersed in the vehicle, which contain iron as a main component, e.g., acicular hematite particles or acicular iron oxide hydroxide particles (hereinafter referred to merely as “non-magnetic undercoat layer”). Such a non-magnetic substrate is already put into practice (refer to Japanese Patent Publication (KOKOKU) No. 6-93297(1994), Japanese Patent Application Laid-Open (KOKAI) Nos. 62-159338(1987), 63-187418(1988), 4-167225(1992), 4-325915(1992), 5-73882(1993), 5-182177(1993), 5-347017(1993) and 6-60362(1994), or the like).

As non-magnetic particles for the non-magnetic undercoat layer, there have been known non-magnetic particles which are treated with a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon or an oxide of silicon in order to improve a dispersibility of these particles in vehicle, etc., for further smoothening the surface of a substrate and increasing the stiffness thereof (Japanese Patent Nos. 2,571,350 and 2,582,051, and Japanese Patent Application Laid-Open (KOKAI) Nos. 6-60362(1994), 9-22524(1997) and 9-27117(1997)).

Also, in order to reduce a light transmittance of magnetic recording media by reducing the amount of carbon black added to a magnetic recording layer thereof, it has been known to use as non-magnetic particles for non-magnetic undercoat layer, blackish brown acicular hematite particles or blackish brown acicular iron oxide hydroxide particles (Japanese Patent Application Laid-Open (KOKAI) Nos. 7-66020(1995), 8-259237(1996) and 9-167333(1997) or the like). Further, it has been known to use non-magnetic acicular particles which are composed of acicular hematite particles or acicular iron oxide hydroxide particles as core particles and carbon black fine particles adhered onto the surfaces of the core particles in an amount of 1 to 20 parts by weight based on 100 parts by weight of the core particles (European Patent No. 0,824,690 A).

Also, it has been known that mixed particles composed of non-magnetic iron oxide particles and carbon black particles are used as non-magnetic particles for non-magnetic undercoat layer in order to reduce the surface electrical resistivity value of magnetic recording media (Japanese Patent Application Laid-Open (KOKAI) Nos. 1-213822(1989), 1-300419(1989), 6-236542(1994) and 9-297911(1997) or the like).

At present, it has been strongly demanded to provide magnetic recording media which exhibit not only a smooth surface, a smaller light transmittance and a lower surface electrical resistivity value, but also a small friction coefficient and an excellent running durability. However, magnetic recording media satisfying all of these properties have not been obtained until now.

As a result of the present inventors' earnest studies in view of the above problems, it has been found that by using as magnetic particles for magnetic recording medium, black magnetic acicular composite particles comprising:

-   -   magnetic acicular cobalt-coated iron oxide particles or magnetic         acicular metal particles containing iron as a main component;     -   a coating formed on surface of the magnetic acicular particle,         comprising at least one organosilicon compound selected from the         group consisting of:     -   (1) organosilane compounds obtainable from alkoxysilane         compounds, and     -   (2) polysiloxane or modified polysiloxane; and     -   a carbon black coat formed on the coating layer comprising the         organosilicon compound, in an amount of from more than 10 to 40         parts by weight based on 100 parts by weight of said magnetic         acicular particles,     -   the obtained magnetic recording medium can exhibit not only a         smooth surface, a smaller light transmittance and a lower         surface electrical resistivity value, but also a small friction         coefficient and an excellent running durability. The present         invention has been attained based on this finding.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a magnetic recording medium exhibiting not only a smooth surface, a smaller light transmittance and a lower surface electrical resistivity value, but also a small friction coefficient and an excellent running durability.

Another object of the present invention is to provide magnetic acicular particles exhibiting not only an excellent dispersibility in vehicle due to a less amount of carbon black desorbed or fallen-off from the surfaces thereof, but also a more excellent blackness, a lower volume resistivity value and a well-controlled myristic acid-adsorption.

To accomplish the aims, in a first aspect of the present invention, there is provided a magnetic recording medium comprising:

-   -   a non-magnetic base film; and     -   a magnetic recording layer formed on the non-magnetic base film,         comprising a binder resin and black magnetic acicular composite         particles having an average particle diameter of 0.051 to 0.35         μm, which black magnetic acicular composite particles comprise:     -   as magnetic acicular particles, magnetic acicular cobalt-coated         iron oxide particles or magnetic acicular metal particles         containing iron as a main component,     -   a coating formed on surface of the magnetic acicular particles,         comprising at least one organosilicon compound selected from the         group consisting of:     -   (1) organosilane compounds obtainable from alkoxysilane         compounds, and     -   (2) polysiloxanes or modified polysiloxanes and     -   a carbon black coat formed on the coating layer comprising the         organosilicon compound, in an amount from more than 10 to 40         parts by weight based on 100 parts by weight of said magnetic         acicular particles.

In a second aspect of the present invention, there is provided a magnetic recording medium comprising:

-   -   a non-magnetic base film; and     -   a magnetic recording layer formed on the non-magnetic base film,         comprising a binder resin and black magnetic acicular composite         particles having an average particle diameter of 0.051 to 0.35         μm, which black magnetic acicular composite particles comprise:     -   as magnetic acicular particles, magnetic acicular cobalt-coated         iron oxide particles or magnetic acicular metal particles         containing iron as a main component, which have a coat formed on         at least a part of the surface of the magnetic acicular         particle, comprising at least one compound selected from the         group consisting of hydroxides of aluminum, oxides of aluminum,         hydroxides of silicon and oxides of silicon in amount of 0.01 to         20% by weight, calculated as Al or SiO₂, based on the weight of         the magnetic acicular particles coated,     -   a coating layer formed on surface of the coat on the magnetic         acicular particles, comprising at least one organosilicon         compound selected from the group consisting of:     -   (1) organosilane compounds obtained obtainable from alkoxysilane         compounds, and     -   (2) polysiloxanes or modified polysiloxanes, and     -   a carbon black coat formed on the said coating layer comprising         the said organosilicon compound, in an amount of from more than         10 to 40 parts by weight based on 100 parts by weight of the         said magnetic acicular particles.

In a third aspect of the present invention, there is provided a magnetic recording medium comprising:

-   -   a non-magnetic base film;     -   a non-magnetic undercoat layer formed on the non-magnetic base         film; and     -   a magnetic recording layer formed on the non-magnetic undercoat         layer, comprising a binder resin and black magnetic acicular         composite particles having an average particle diameter of 0.051         to 0.35 μm, which black magnetic acicular composite particles         comprise:     -   as magnetic acicular particles, magnetic acicular cobalt-coated         iron oxide particles or magnetic acicular metal particles         containing iron as a main component,     -   a coating formed on surface of said magnetic acicular particles,         comprising at least one organosilicon compound selected from the         group consisting of:     -   (1) organosilane compounds obtainable from alkoxysilane         compounds, and     -   (2) polysiloxanes or modified polysiloxanes, and     -   a carbon black coat formed on the coating layer comprising said         organosilicon compound, in an amount of from more than 10 to 40         parts by weight based on 100 parts by weight of the magnetic         acicular particles.

In a fourth aspect of the present invention, there is provided a magnetic recording medium comprising:

-   -   a non-magnetic base film;     -   a non-magnetic undercoat layer formed on the non-magnetic base         film; and     -   a magnetic recording layer formed on the non-magnetic undercoat         layer, comprising a binder resin and black magnetic acicular         composite particles having an average particle diameter of 0.051         to 0.35 μm, which black magnetic acicular composite particles         comprise:     -   as magnetic acicular particles, magnetic acicular cobalt-coated         iron oxide particles or magnetic acicular metal particles         containing iron as a main component, which have a coat formed on         at least a part of the surface of the magnetic acicular         particle, comprising at least one compound selected from the         group consisting of hydroxides of aluminum, oxides of aluminum,         hydroxides of silicon and oxides of silicon in amount of 0.01 to         20% by weight, calculated as Al or SiO₂, based on the weight of         the magnetic acicular particles coated,     -   a coating layer formed on surface of the coat on the magnetic         acicular particles, comprising at least one organosilicon         compound selected from the group consisting of:     -   (1) organosilane compounds obtained obtainable from alkoxysilane         compounds, and     -   (2) polysiloxanes or modified polysiloxanes, and a carbon black         coat formed on the said coating layer comprising the said         organosilicon compound, in an amount of from more than 10 to 40         parts by weight based on 100 parts by weight of the said         magnetic acicular particles

In a fifth aspect of the present invention, there is provided a magnetic recording medium comprising:

-   -   a non-magnetic base film;     -   a non-magnetic undercoat layer formed on the non-magnetic base         film; and     -   a magnetic recording layer formed on the non-magnetic undercoat         layer, comprising a binder resin and black magnetic acicular         composite particles having an average particle diameter of 0.051         to 0.35 μm, which black magnetic acicular composite particles         comprise:     -   as magnetic acicular particles, magnetic acicular cobalt-coated         iron oxide particles or magnetic acicular metal particles         containing iron as a main component,     -   a coating formed on surface of said magnetic acicular particles,         comprising at least one organosilicon compound selected from the         group consisting of:     -   (1) organosilane compounds obtainable from alkoxysilane         compounds, and     -   (2) polysiloxanes or modified polysiloxanes, and     -   a carbon black coat formed on said coating layer comprising said         organosilicon compound, in an amount of 0.5 to 10 parts by         weight based on 100 parts by weight of said magnetic acicular         particles.

In a sixth aspect of the present invention, there is provided a magnetic recording medium comprising:

-   -   a non-magnetic base film;     -   a non-magnetic undercoat layer formed on the non-magnetic base         film; and     -   a magnetic recording layer formed on the non-magnetic undercoat         layer, comprising a binder resin and black magnetic acicular         composite particles having an average particle diameter of 0.051         to 0.35 μm, which black magnetic acicular composite particles         comprise:     -   as magnetic acicular particles, magnetic acicular cobalt-coated         iron oxide particles or magnetic acicular metal particles         containing iron as a main component, which have a coat formed on         at least a part of the surface of the magnetic acicular         particle, comprising at least one compound selected from the         group consisting of hydroxides of aluminum, oxides of aluminum,         hydroxides of silicon and oxides of silicon in amount of 0.01 to         20% by weight, calculated as Al or SiO₂, based on the weight of         the magnetic acicular particles coated,     -   a coating layer formed on surface of the coat on the magnetic         acicular particles, comprising at least one organosilicon         compound selected from the group consisting of:     -   (1) organosilane compounds obtained obtainable from alkoxysilane         compounds, and     -   (2) polysiloxanes or modified polysiloxanes, and     -   a carbon black coat formed on the said coating layer comprising         the said organosilicon compound, in an amount of 0.5 to 10 parts         by weight based on 100 parts by weight of the said magnetic         acicular particles

In a seventh aspect of the present invention, there are provided black magnetic acicular composite particles for a magnetic recording medium, having an average particle diameter of 0.051 to 0.35 μm, comprising:

-   -   as magnetic acicular particles, magnetic acicular cobalt-coated         iron oxide particles or magnetic acicular metal particles         containing iron as a main component,     -   a coating formed on surface of said magnetic acicular particles,         comprising at least one organosilicon compound selected from the         group consisting of:     -   (1) organosilane compounds obtainable from alkoxysilane         compounds, and     -   (2) polysiloxanes or modified polysiloxanes, and     -   a carbon black coat formed on said coating layer comprising said         organosilicon compound, in an amount of from more than 10 to 40         parts by weight based on 100 parts by weight of said magnetic         acicular particles.

In an eighth aspect of the present invention, there are provided black magnetic acicular composite particles for a magnetic recording medium, having an average particle diameter of 0.051 to 0.35 μm, comprising:

-   -   as magnetic acicular particles, magnetic acicular cobalt-coated         iron oxide particles or magnetic acicular metal particles         containing iron as a main component, which have a coat formed on         at least a part of the surface of the magnetic acicular         particle, comprising at least one compound selected from the         group consisting of hydroxides of aluminum, oxides of aluminum,         hydroxides of silicon and oxides of silicon in amount of 0.01 to         20% by weight, calculated as Al or SiO₂, based on the weight of         the magnetic acicular particles coated,     -   a coating layer formed on surface of the coat on the magnetic         acicular particles, comprising at least one organosilicon         compound selected from the group consisting of:     -   (1) organosilane compounds obtainable from alkoxysilane         compounds, and     -   (2) polysiloxanes or modified polysiloxanes, and     -   a carbon black coat formed on the coating layer comprising said         organosilicon compound, in an amount of from more than 10 to 40         parts by weight based on 100 parts by weight of the said         magnetic acicular particles.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail below.

First, the magnetic recording medium according to the present invention is described.

The magnetic recording medium according to the present invention has the following constructions.

(1) A magnetic recording medium comprises a non-magnetic base film; and a magnetic recording layer formed on the non-magnetic base film, which comprises as magnetic particles, black magnetic acicular composite particles and a binder resin.

(2) A magnetic recording medium comprises a non-magnetic base film; a non-magnetic undercoat layer formed on the non-magnetic base film, which comprises non-magnetic particles and a binder resin; and a magnetic recording layer formed on the non-magnetic undercoat layer, which comprises as magnetic particles, black magnetic acicular composite particles and a binder resin.

The black magnetic acicular composite particles having an average particle diameter of 0.051 to 0.35 μm according to the present invention, comprise magnetic acicular particle as a core particle, a coating layer comprising an organosilicon compound which is formed on the surface of each magnetic acicular particle, and carbon black coat which are formed on the coating layer comprising the organosilicon compound.

As the magnetic acicular particles used as core particles in the present invention, there may be exemplified (a) magnetic acicular cobalt-coated iron oxide particles obtained by coating with cobalt, both cobalt and iron or the like the surface of as acicular magnetite particles (Fe₃O₄), acicular maghemite particles (γ-Fe₂O₃), acicular berthollide compounds particles (FeO _(x).Fe₂O₃; 0<X<1) which are intermediate oxides between maghemite and magnetite (hereinafter referred to merely as “magnetic acicular cobalt-coated iron oxide particles”); magnetic acicular metal particles containing iron as a main component which contain elements other than Fe such as Co, Al, Ni, P, Zn, Si, B or rare earth elements (hereinafter referred to merely as “magnetic acicular metal particles”) (which may include magnetic acicular iron alloy particles containing elements other than Fe such as Co, Al, Ni, P, Zn, Si, B or rare earth elements); or the like.

More specifically, the magnetic acicular metal particles may be exemplified as follows.

1) Magnetic acicular metal particles comprises iron; and cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles.

2) Magnetic acicular metal particles comprises iron; and aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles.

3) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; and aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles.

4) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.

5) Magnetic acicular metal particles comprises iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.

6) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.

7) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.

8) Magnetic acicular metal particles comprises iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.

9) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 3.5% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.

10) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.

11) Magnetic acicular metal particles comprises iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; at least one selected from the group consisting of Nd, La and Y of ordinarily 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.

12) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.

The iron content in the magnetic acicular metal particles is the balance, and is preferably 50 to 99% by weight, more preferably 60 to 95% by weight (calculated as Fe) based on the weight of the magnetic acicular metal particles.

The core particles used in the present invention have an acicular shape. Meanwhile, the “acicular” used herein means “spindle-shaped”, “rice grain-like” or the like in addition to literally “acicular or needle-like”.

As to the particle size of the magnetic acicular particles used in the present invention, the average major axial diameter thereof is preferably 0.05 to 0.34 μm, more preferably 0.05 to 0.33 μm, still more preferably 0.05 to 0.32 μm.

When the average major axis diameter of the magnetic acicular particles is more than 0.34 μm, the obtained black magnetic acicular composite particles also may become large particles. In the case where such large particles are used for forming a magnetic recording layer, the surface smoothness of the magnetic recording layer tends to be deteriorated on the other hand, when the average particle size is less than 0.05 μm, the intermolecular force between the particles may be increased due to the reduction in particle size, so that agglomeration of the particles tends to be caused. Therefore, it becomes difficult to uniformly coat the surfaces of the magnetic acicular particle with the organosilicon compound, and to uniformly form the carbon black coat on the surface of the coating layer comprising the organosilicon compounds.

The average minor axis diameter of the magnetic acicular particles as core particles used in the present invention is preferably 0.0025 to 0.17 μm, more preferably 0.0025 to 0.165 μm, still more preferably 0.0025 to 0.16 μm.

The aspect ratio of the magnetic acicular particles as core particles used in the present invention (=average major axis diameter:average minor axis diameter, hereinafter referred to merely as “aspect ratio”) is usually 2.0:1 to 20.0:1, preferably 2.5:1 to 18.0:1, more preferably 3.0:1 to 15.0:1. When the aspect ratio is more than 20.0:1, the magnetic acicular particles may tend to be entangled with each other, and it also may become difficult to uniformly coat the surfaces of the magnetic acicular particles with the organosilicon compounds and to uniformly form the carbon black coat on the surface of the coating layer comprising the organosilicon compounds. On the other hand, when the aspect ratio is less than 2.0:1, the strength of the coating film of the magnetic recording medium may be low.

The geometrical standard deviation value of the major axis diameter of the magnetic acicular particles as core particles used in the present invention is usually not more than 2.0, preferably not more than 1.8, more preferably not more than 1.6. When the geometrical standard deviation value thereof is more than 2.0, coarse particles may be contained therein, so that the magnetic acicular particles may be inhibited from being uniformly dispersed. Therefore, it may also become difficult to uniformly coat the surfaces of the magnetic acicular particles with the organosilicon compounds and to uniformly form the carbon black coat on the surface of the coating layer comprising the organosilicon compounds. The lower limit of the geometrical standard deviation value is 1.01. It is industrially difficult to obtain particles having a geometrical standard deviation value of less than 1.01.

The BET specific surface area of the magnetic acicular particles as core particles used in the present invention is usually 20 to 150 m²/g, preferably 25 to 120 m²/g, more preferably 28 to 100 m²/g. When the BET specific surface area is less than 20 m²/g, the magnetic acicular particles may become coarse, or the sintering between the particles may be caused, so that the obtained black magnetic acicular composite particles also may become coarse particles and tend to be deteriorated in smooth surface of the magnetic recording layer. When the BET specific surface area value is more than 150 m²/g, the particles may tend to be agglomerated together due to the increase in intermolecular force between the particles because of the fineness thereof, so that it may become difficult to uniformly coat the surfaces of the magnetic acicular particles with the organosilane compounds and to uniformly form the carbon black coat on the surface of the coating layer comprising the organosilicon compounds.

The blackness of the magnetic acicular particles as core particles used in the present invention, is usually more than 18.0 when represented by L* value, and the upper limit thereof is usually 34.0, preferably 32.0 when represented by L* value. When the L* value exceeds 34.0, the lightness of the particles may be high, so that it may be difficult to obtain black magnetic acicular composite particles having a sufficient blackness.

The volume resistivity value of the core particles is usually not less than 5×10¹⁰ Ω·cm, preferably 1×10⁶ to 1×10¹⁰ Ω·cm.

The myristic acid-adsorption of the core particles is usually 0.4 to 1.5, preferably 0.4 to 1.2 mg/m².

As to the magnetic properties of the magnetic acicular particles as core particles used in the present invention, the coercive force value thereof is usually 500 to 3500 Oe (39.8 to 278.5 kA/m), the saturation magnetization value is usually 60 to 170 emu/g (60 to 170 Am²/kg).

In case of magnetic acicular cobalt-coated iron oxide particles, the coercive force value thereof is usually 500 to 1700 Oe (39.8 to 135.3 kA/m), preferably 550 to 1700 Oe (43.8 to 135.3 kA/m); the saturation magnetization value is usually 60 to 90 emu/g (60 to 90 Am²/kg), preferably 65 to 90 emu/g (65 to 90 Am²/kg). In case of magnetic acicular metal particles containing iron as a main component, the coercive force value thereof is usually 800 to 3500 Oe (63.7 to 278.5 kA/m), preferably 900 to 3500 Oe (71.6 to 278.5 kA/m); the saturation magnetization value is usually 90 to 170 emu/g (90 to 170 Am²/kg), preferably 100 to 170 emu/g (100 to 170 Am²/kg).

The coating formed on the surface of the core particle comprises at least one organosilicon compound selected from the group consisting of (1) organosilane compounds obtainable from alkoxysilane compounds; and (2) polysiloxanes and modified polysiloxanes selected from the group consisting of (2-A) polysiloxanes modified with at least one compound selected from the group consisting of polyethers, polyesters and epoxy compounds (hereinafter referred to merely as “modified polysiloxanes”), and (2-B) polysiloxanes whose molecular terminal is modified with at least one group selected from the group consisting of carboxylic acid groups, alcohol groups and a hydroxyl group (hereinafter referred to merely as “terminal-modified polysiloxanes”).

The organosilane compounds (1) may be produced by drying or heat-treating alkoxysilane compounds represented by the formula (I): R¹ _(a)SiX_(4−a)  (I) wherein R¹ is C₆H₅—, (CH₃)₂CHCH₂— or n-C_(b)H_(2b+1)— (wherein b is an integer of 1 to 18); X is CH3O— or C₂H₅O—; and a is an integer of 0 to 3.

The drying or heat-treatment of the alkoxysilane compounds may be conducted, for example, at a temperature of usually 40 to 200° C., preferably 60 to 150° C. for usually 10 minutes to 12 hours, preferably 30 minutes to 3 hours.

Specific examples of the alkoxysilane compounds may include methyltriethoxysilane, dimethyldiethoxysilane, phenyltriethyoxysilane, diphenyldiethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, diphenyldimethoxysilane, isobutyltrimethoxysilane, decyltrimethoxysilane or the like. Among these alkoxysilane compounds, in view of the desorption percentage and the adhering effect of carbon black, methyltriethoxysilane, phenyltriethyoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane and isobutyltrimethoxysilane are preferred, and methyltriethoxysilane and methyltrimethoxysilane are more preferred.

As the polysiloxanes (2), there may be used those compounds represented by the formula (II):

wherein R² is H— or CH₃—, and d is an integer of 15 to 450.

Among these polysiloxanes, in view of the desorption percentage and the adhering effect of carbon black, polysiloxanes having methyl hydrogen siloxane units are preferred.

As the modified polysiloxanes (2-A), there may be used:

-   (a) polysiloxanes modified with polyethers represented by the     formula (III):     wherein R³ is —(—CH₂—)_(h)—; R⁴ is —(—CH₂—)_(i)—CH₃; R⁵ is —OH,     —COOH, —CH═CH₂, —C(CH₃)═CH₂ or —(—CH₂—)_(j)—CH₃; R⁶ is     —(—CH₂—)_(k)—CH₃; g and h are an integer of 1 to 15; i, j and k are     an integer of 0 to 15; e is an integer of 1 to 50; and f is an     integer of 1 to 300; -   (b) polysiloxanes modified with polyesters represented by the     formula (IV):     wherein R⁷, R⁸ and R⁹ are —(—CH₂—)_(q)— and may be the same or     different; R¹⁰ is —OH, —COOH, —CH═CH₂, —C(CH₃)═CH₂ or     —(—CH₂—)_(r)—CH₃; R¹¹ is —(—CH₂—)_(s)—CH₃; n and q are an integer of     1 to 15; r and s are an integer of 0 to 15; e′ is an integer of 1 to     50; and f′ is an integer of 1 to 300; -   (c) polysiloxanes modified with epoxy compounds represented by the     formula (V):     wherein R¹² is —(—CH₂—)_(v)—; v is an integer of 1 to 15; t is an     integer of 1 to 50; and u is an integer of 1 to 300; or a mixture     thereof.

Among these modified polysiloxanes (2-A), in view of the desorption percentage and the adhering effect of carbon black, the polysiloxanes modified with the polyethers represented by the formula (III), are preferred.

As the terminal-modified polysiloxanes (2-B), there may be used those represented by the formula (VI):

wherein R¹³ and R¹⁴ are —OH, R¹⁶OH or R¹⁷COOH and may be the same or different; R¹⁵ is —CH₃ or —C₆H₅; R¹⁶ and R¹⁷ are —(—CH₂—)_(y)—; y is an integer of 1 to 15; w is an integer of 1 to 200; and x is an integer of 0 to 100.

Among these terminal-modified polysiloxanes, in view of the desorption percentage and the adhering effect of carbon black, the polysiloxanes whose terminals are modified with carboxylic acid groups are preferred.

The coating amount of the organosilicon compounds is usually 0.02 to 5.0% by weight, preferably 0.03 to 4.0% by weight, more preferably 0.05 to 3.0% by weight (calculated as Si) based on the weight of the magnetic acicular particles coated with the organosilicon compounds.

When the coating amount of the organosilicon compounds is less than 0.02% by weight, it becomes difficult to coat the carbon black on the surfaces of the magnetic acicular particles.

On the other hand, when the coating amount of the organosilicon compounds is more than 5.0% by weight, the carbon black coat in an amount of 0.5 to 10 parts by weight based on 100 parts by weight of the core particles, can be formed on the surfaces of the magnetic acicular particles.

The total amount of the carbon black coat formed on the coating layer composed of the organosilicon compounds is usually 0.5 to 40 parts by weight. Specially, (i) the amount of the carbon black coat formed on the coating layer composed of the organosilicon compounds is usually 0.5 to 10 parts by weight based on 100 parts by weight of the magnetic acicular particles as the core particles; and (ii) the amount of the carbon black coat formed on the coating layer composed of the organosilicon compounds is usually from more than 10 to 40 parts by weight based on 100 parts by weight of the magnetic acicular particles as the core particles. Detailedly, in case of from more than 10 to 40 parts by weight of the carbon black coat, the amount of the carbon black coat (hereinafter referred to merely as “first carbon black coat”) formed on the coating layer composed of the organosilicon compounds is usually 0.5 to 10 parts by weight based on 100 parts by weight of the magnetic acicular particles as the core particles, and the amount of a layer composed of carbon black which is formed onto the surface of the first carbon black coat (hereinafter referred to merely as “second carbon black coat”) through dimethylpolysiloxanes is usually 1 to 30 parts by weight based on 100 parts by weight of the core particles.

When the amount of the carbon black coat formed is less than 0.5 part by weight, the blackness of the resultant black magnetic acicular composite particles may be unsatisfactory because of insufficient amount of the carbon black coat formed onto the coating layer.

When the total amount of carbon black coat formed is more than 40 parts by weight, the effects of improving the blackness and the volume resistivity value of the obtained black magnetic acicular composite particles are already saturated and, therefore, the adhesion of such a large amount of carbon black is unnecessary and meaningless.

When the amount of carbon black coat is large, especially, (ii) more than 10 parts by weight, there can be obtained black acicular composite particles exhibiting a well-controlled myristic acid-adsorption. More specifically, such black magnetic acicular composite particles on which carbon black coat is formed in an amount of (i) not more than 10 parts by weight based on 100 parts by weight of the core particles, exhibit a myristic acid-adsorption as high as more than 0.34 mg/m². On the contrary, the black acicular composite particles on which carbon black coat is formed in an amount of more than 10 parts by weight based on 100 parts by weight of the core particles of the seven aspect can exhibit a good myristic acid-adsorption of not more than 0.30 mg/m².

As the carbon black fine particles used in the present invention, there may be exemplified commercially available carbon blacks such as furnace black, channel black or the like. Specific examples of the commercially available carbon blacks usable in the present invention, may include #3050, #3150, #3250, #3750, #3950, MA100, MA7, #1000, #2400B, #30, MA77, MA8, #650, MA11, #50, #52, #45, #2200B, MA600, etc. (tradename, produced by MITSUBISHI CHEMICAL CORP.), SEAST 9H, SEAST 7H, SEAST 6, SEAST 3H, SEAST 300, SEAST FM, etc. (tradename, produced by TOKAI CARBON CO., LTD.), Raven 1250, Raven 860, Raven 1000, Raven 1190 ULTRA, etc. (tradename, produced by COLOMBIAN CHEMICALS COMPANY), Ketchen black EC, Ketchen black EC600JD, etc. (tradename, produced by KETCHEN INTERNATIONAL CO., LTD.), BLACK PEARLS-L, BLACK PEARLS 1000, BLACK PEARLS 4630, VULCAN XC72, REGAL 660, REGAL 400, etc. (tradename, produced by CABOTT SPECIALTY CHEMICALS INK CO., LTD.), or the like.

In the consideration of the reducing effect of the myristic acid-adsorption, the carbon black fine particles having a pH value of not more than 9.0 is preferred. Especially, there may be exemplified #3050, #3150, #3250, #3750, #3950, MA100, MA7, #1000, #2400B, #30, MA77, MA8, #650, MA11, #50, #52, #45, #2200B, MA600 (tradename, produced by MITSUBISHI CHEMICAL CORP.), SEAST 9H, SEAST 7H, SEAST 6, SEAST 3H, SEAST 300, SEAST FM (tradename, produced by TOKAI CARBON CO., LTD.), Raven 1250, Raven 860, Raven 1000, Raven 1190 ULTRA (tradename, produced by COLOMBIAN CHEMICALS COMPANY), BLACK PEARLS-L, BLACK PEARLS 1000, BLACK PEARLS 4630, REGAL 660, REGAL 400 (tradename, produced by CABOT SPECIALTY CHEMICALS INK CO., LTD.).

Further, in the consideration of more uniform coat of carbon black to the coating layer comprising at least one organosilicon compound, the carbon black fine particles having a DBP oil absorption of not more than 180 ml/100 g is preferred. Especially, there may be exemplified #3050, #3150, #3250, MA100, MA7, #1000, #2400B, #30, MA77, MA8, #650, MA11, #50, #52, #45, #2200B, MA600 (tradename, produced by MITSUBISHI CHEMICAL CORP.), SEAST 9H, SEAST 7H, SEAST 6, SEAST 3H, SEAST 300, SEAST FM (tradename, produced by TOKAI CARBON CO., LTD.), Raven 1250, Raven 860, Raven 1000, Raven 1190 ULTRA (tradename, produced by COLOMBIAN CHEMICALS COMPANY), BLACK PEARLS-L, BLACK PEARLS 1000, BLACK PEARLS 4630, REGAL 660, REGAL 400 (tradename, produced by CABOT SPECIALTY CHEMICALS INK CO., LTD.).

The lower limit of the average particle size of the carbon black fine particles used is usually 0.002 μm, preferably 0.005 μm, and upper limit thereof is usually 0.05 μm, preferably 0.035 μm. When the average particle size of the carbon black fine particles used is less than 0.002 μm, the carbon black fine particles used are too fine to be well handled.

On the other hand, when the average particle size of the carbon black fine particles used is more than 0.05 μm, since the carbon black fine particles used is much larger, it is necessary to apply a larger mechanical shear force for forming the uniform carbon black coat on the coating layer composed of the organosilicon compounds, thereby rendering the coating process industrially disadvantageous.

In the present invention, when the amount of carbon black coat is large, especially more than 10 parts by weight based on 100 parts by weight of the core particles, at least two carbon black coats bonded together are formed onto the coating of organosilicon compounds.

In the consideration of the desorption percentage and adhering effect of carbon black, (i) the amount of the first carbon black coat formed onto the coating of organosilicon compounds is usually 0.5 to 10 parts by weight, preferably 1 to 10 parts by weight, more preferably 3 to 10 parts by weight based on 100 parts by weight of the core particles; and (ii) the amount of the second carbon black coat formed onto the surface of the first carbon black coat through adhesives is usually 1 to 30 parts by weight, preferably 5 to 25 parts by weight based on 100 parts by weight of the core particles. Further, in the consideration of the myristic acid-adsorption and carbon black desorption percentage, the total amount of carbon black coat is preferably 11 to 38 parts by weight, more preferably 12 to 35 parts by weight based on 100 parts by weight of the core particles.

In the present invention, the first and second carbon black coats may be integrated together by bonding carbon black of the respective coats with each other using an adhesive. In order to firmly and uniformly bond the carbon black coats with each other and control the myristic acid-adsorption to an appropriate level, it is preferred to use as the adhesive, dimethyl polysiloxane represented by the following formula:

wherein v′ is a is an integer of 15 to 450.

The amount of the adhesive-used is 0.1 to 5 parts by weight based on 100 parts by weight of the core particles.

When the amount of the adhesive used is less than 0.1 part by weight, it may be difficult to sufficiently bond the second carbon black coat onto the first carbon black coat. As a result, it may be difficult to improve the myristic acid-adsorption of the obtained black magnetic acicular composite particles.

When the amount of the adhesive used is more than 5 parts by weight, although the carbon black coats can be sufficiently bonded together, the effect is already saturated and, therefore, the use of such a large amount of the adhesive is unnecessary and meaningless.

When the amount of carbon black coat is in the range of 0.5 to 10 parts by weight based on 100 parts by weight of the core particles, the thickness of carbon black coat is preferably not more than 0.04 μm, more preferably not more than 0.02 μm, still more preferably not more than 0.01 μm.

When the amount of carbon black coat is in the range of from more than 10 to 40 parts by weight based on 100 parts by weight of the core particles, the thickness of carbon black coat is usually not more than 0.06 μm, preferably not more than 0.04 μm, more preferably not more than 0.03 μm, still more preferably not more than 0.02 μm.

The particle shape and size of the black magnetic acicular composite particles according to the present invention, are considerably varied depending upon those of the core particles. More specifically, the particle shape or configuration of the magnetic acicular composite particles is similar to that of the core particles, and the particle size of the magnetic acicular composite particles is slightly larger than that of the core particles.

That is, the black magnetic acicular composite particles according to the present invention, have an average major axial diameter of usually 0.051 to 0.35 μm, preferably 0.051 to 0.34 μm, more preferably 0.051 to 0.33 μm; and an aspect ratio of usually 2.0:1 to 20.0:1, preferably 2.5:1 to 18.0:1, more preferably 3.0:1 to 15.0:1.

When the average major axial diameter is more than 0.35 μm, the obtained black magnetic acicular composite particles become too large. In the case where such large particles are used, the obtained magnetic recording layer may tend to be deteriorated in surface smoothness of coating film. When the average major axial diameter is less than 0.051 μm, the intermolecular force between the particles may become too large due to fineness thereof, so that the obtained composite particles may tend to be agglomerated together, resulting in deteriorated dispersibility in vehicle upon the production of a magnetic coating composition.

When the aspect ratio is more than 20.0:1, the particles may tend to be entangled or interlaced with each other, sometimes resulting in deteriorated dispersibility in vehicle upon the production of a magnetic coating composition and increased viscosity of the magnetic coating composition. When the aspect ratio is less than 2.0:1, the strength of coating film of the obtained magnetic recording medium using such particles may become small.

The geometrical standard deviation value of major axial diameters of the black magnetic acicular composite particles according to the present invention is usually not more than 2.0. When the geometrical standard deviation value is more than 2.0, coarse particles may be present in the obtained magnetic acicular composite particles, thereby adversely affecting the surface smoothness of the obtained coating film. In the consideration of the surface smoothness of coating film, the geometrical standard deviation value is preferably not more than 1.8, more preferably not more than 1.6. In the consideration of industrial productivity, the geometrical standard deviation value is not less than 1.01. Meanwhile, the particles having a geometrical standard deviation value of less than 1.01 are difficult to industrially produce.

The black magnetic acicular composite particles of the seventh and eighth aspects, have a myristic acid absorption of usually 0.01 to 0.3 mg/m², preferably 0.01 to 0.29 mg/m², more preferably 0.01 to 0.28 mg/m².

When the myristic acid-adsorption of the black magnetic acicular composite particles is within the above-specified range, the amount of myristic acid absorbed thereinto is adequate, so that the amount of myristic acid oozed onto the surface of the magnetic recording layer can be readily controlled to an appropriate level. As a result, upon the repeated use of magnetic tape, it is possible to maintain a sufficiently low friction coefficient of the magnetic tape for a long period of time, thereby ensuring an excellent running property thereof.

The black magnetic acicular composite particles according to the present invention have a BET specific surface area of preferably 21 to 160 m²/g, more preferably 26 to 130 m²/g, still more preferably 29 to 110 m²/g. When the BET specific surface area is less than 21 m²/g, the obtained black magnetic acicular composite particles may tend to become coarse or be sintered together. In the case where such particles are used to produce a magnetic recording layer, the obtained coating film may tend to be deteriorated in surface smoothness. When the BET specific surface area is more than 160 m²/g, the intermolecular force between the particles may be increased due to fineness thereof. As a result, the obtained particles may tend to be agglomerated together, resulting in deteriorated dispersibility in vehicle upon the production of magnetic coating composition.

As to the blackness of the black magnetic acicular composite particles having 0.5 to 10 parts by weight of the carbon black coat, the L* value thereof is usually not more than 23, preferably not more than 22, more preferably not more than 21. When the L* value is more than 23, the lightness of the particles becomes too high, thereby failing to obtain particles having a sufficient blackness. The L* value of the black magnetic acicular composite particles thereof is not less than 15.

The black magnetic acicular composite particles having 0.5 to 10 parts by weight of the carbon black coat, have a volume resistivity value of preferably not more than 1.0×10⁷ Ω·cm, more preferably 1.0×10³ to 5.0×10⁶ Ω·cm, still more preferably 1.0×10³ to 1.0×10⁶ Ω·cm. When the volume resistivity value is more than 1.0×10⁷ Ω·cm, it is difficult to sufficiently reduce the surface electrical resistivity value of the obtained magnetic recording medium.

As to the blackness of the black magnetic acicular particles of the seven and eighth aspects, the L* value thereof is preferably not more than 22, more preferably not more than 21, still more preferably not more than 20. When the L* value is more than 22, the lightness of the particles becomes too high, thereby failing to obtain particles having a sufficient blackness. The L* value of the black magnetic acicular particles thereof is not less than 15.

The black magnetic acicular particles of the seventh and eighth aspects have a volume resistivity value of preferably not more than 1.0×10⁶ Ω·cm, more preferably 1.0×10³ to 5.0×10⁵ Ω·cm, still more preferably 1.0×10³ to 1.0×10⁵ Ω·cm. When the volume resistivity value is more than 1.0×10⁶ Ω·cm, it is difficult to sufficiently reduce the surface electrical resistivity value of the obtained magnetic recording medium.

The carbon black desorption percentage of the black magnetic acicular composite particles is preferably not more than 20%, more preferably not more than 10%. When the carbon black desorption percentage is more than 20%, the obtained particles may tend to be inhibited from uniformly dispersed in vehicle due to desorbed and liberated carbon black upon the production of magnetic recording media.

As to magnetic properties of the black magnetic acicular composite particles according to the present invention, the coercive force value thereof is usually 500 to 3500 Oe (39.8 to 278.5 kA/m), the saturation magnetization value is usually 60 to 170 emu/g (60 to 170 Am²/kg).

In the case where the magnetic acicular cobalt-coated iron oxide particles are used as the core particles, the coercive force value thereof is preferably 500 to 1,700 Oe (39.8 to 135.3 kA/m), more preferably 550 to 1,700 Oe (43.8 to 135.3 kA/m); and the saturation magnetization value thereof is preferably 60 to 90 emu/g (60 to 90 Am²/kg), more preferably 65 to 90 emu/g (65 to 90 Am²/kg).

In the case where the magnetic acicular metal particles containing iron as a main component are used as the core particles, the coercive force value thereof is preferably 800 to 3,500 Oe (63.7 to 278.5 kA/m), more preferably 900 to 3,500 Oe (71.6 to 278.5 kA/m); and the saturation magnetization value thereof is preferably 90 to 170 emu/g (90 to 170 Am²/kg), more preferably 100 to 170 emu/g (100 to 170 Am²/kg).

At least a part of the surface of the magnetic acicular particle as a core particle used in the present invention may be coated with at least one selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon (hereinafter referred to as “hydroxides and/or oxides of aluminum and/or silicon”). When the black magnetic acicular composite particles obtained by using as core particles the magnetic acicular particles which are coated with the hydroxides and/or oxides of aluminum and/or silicon, are dispersed in a vehicle, since it is possible to more effectively reduce the carbon black desorption percentage, it is more easy to obtain a desired dispersibility.

The amount of the hydroxides and/or oxides of aluminum and/or silicon coat is usually not more than 20% by weight, preferably 0.01 to 20% by weight (calculated as Al and/or SiO₂) based on the total weight of the magnetic acicular particles coated. If it is less than 0.01% by weight (calculated as Al and/or SiO₂) based on the total weight of the magnetic acicular particles coated, the dispersibility-improving effect by coating therewith may be insufficient.

When the coating amount of the hydroxide of aluminum or the like is more than 20% by weight, although there can be obtained a sufficient effect of reducing the percentage of carbon black desorbed or fallen-off from the surfaces of the core particles, the effect is already saturated and, therefore, the use of such a large coating amount of the hydroxide of aluminum or the like is unnecessary and meaningless.

From the point of view of dispersibility in the vehicle and industrial productivity, the more preferable amount of coating composition is 0.05 to 10% by weight (calculated as Al and/or SiO₂) based on the total weight of the magnetic acicular particles coated.

The particle size, geometrical standard deviation value, BET specific surface area, blackness L* value, volume resistivity and magnetic properties of the black magnetic acicular composite particles wherein the surface of the core particle is coated with the hydroxides and/or oxides of aluminum and/or silicon according to the present invention, are substantially the same as those of the black magnetic acicular composite particles wherein the core particle is uncoated with the hydroxides and/or oxides of aluminum and/or silicon according to the present invention.

In addition, the desorption percentage of carbon black of the black magnetic acicular composite particles wherein the surface of the core particle is coated with the hydroxides and/or oxides of aluminum and/or silicon is more improved as compared with that of the black magnetic acicular composite particles wherein the core particle is uncoated therewith. The carbon black desorption percentage of the black magnetic acicular composite particles wherein the surface of the core particle is coated with the hydroxides and/or oxides of aluminum and/or silicon, is preferably not more than 10%, more preferably not more than 5%.

Next, the magnetic recording medium of the first and second aspect in the present invention is described.

The magnetic recording medium according to the present invention comprises:

-   -   a non-magnetic base film; and     -   a magnetic recording layer formed on the non-magnetic base film,         comprising a binder resin and the black magnetic acicular         composite particles.

As the non-magnetic base film, the following materials which are at present generally used for the production of a magnetic recording medium are usable as a raw material: a synthetic resin such as polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamideimide and polyimide; foil and plate of a metal such as aluminum and stainless steel; and various kinds of paper. The thickness of the non-magnetic base film varies depending upon the material, but it is usually about 1.0 to 300 μm, preferably 2.0 to 200 μm.

In the case of a magnetic disc, polyethylene terephthalate is usually used as the non-magnetic base film, and the thickness thereof is usually 50 to 300 μm, preferably 60 to 200 μm.

In a magnetic tape, when polyethylene terephthalate is used as the non-magnetic base film, the thickness thereof is usually 3 to 100 μm, preferably 4 to 20 μm; when polyethylene naphthalate is used, the thickness thereof is usually 3 to 50 μm, preferably 4 to 20 μm; and when polyamide is used, the thickness thereof is usually 2 to 10 μm, preferably 3 to 7 μm.

As the binder resin used in the present invention, the following resins which are at present generally used for the production of a magnetic recording medium are usable: vinyl chloride-vinyl acetate copolymer, polyurethane resin, vinyl chloride-vinyl acetate-maleic acid copolymer, urethane elastomer, butadiene-acrylonitrile copolymer, polyvinyl butyral, cellulose derivative such as nitrocellulose, polyester resin, synthetic rubber resin such as polybutadiene, epoxy resin, polyamide resin, polyisocyanate, electron radiation curing acryl urethane resin and mixtures thereof.

Each of these resin binders may contain a functional group such as —OH, —COOH, —SO₃M, —OPO₂M₂ and —NH₂, wherein M represents H, Na or K. With the consideration of the dispersibility of the black magnetic acicular composite particles, a binder resin containing a functional group —COOH or —SO₃M is preferable.

The thickness of the magnetic recording layer obtained by applying the magnetic coating composition on the surface of the non-magnetic base film and dried, is usually in the range of 0.01 to 5.0 μm. If the thickness is less than 0.01 μm, uniform coating may be difficult, so that unfavorable phenomenon such as unevenness on the coating surface is observed. On the other hand, when the thickness exceeds 5.0 μm, it may be difficult to obtain desired signal recording property due to an influence of diamagnetism. The preferable thickness is in the range of 0.1 to 4.0 μm.

The mixing ratio of the black magnetic acicular composite particles with the binder resin is usually 5 to 2000 parts by weight, preferably 100 to 1000 parts by weight based on 100 parts by weight of the binder resin.

When the amount of the black magnetic acicular composite particles blended is less than 5 parts by weight, the obtained magnetic coating composition contains a too small amount of the black magnetic acicular composite particles. As a result, when a coating film is produced from such a magnetic coating composition, it is not possible to obtain a coating film in which the black magnetic acicular composite particles are continuously dispersed, so that the surface smoothness and the strength of the coating film become unsatisfactory. On the other hand, when the amount of the black magnetic acicular composite particles blended is more than 2,000 parts by weight, the amount of the black magnetic acicular composite particles becomes too large relative to that of the binder resin, so that it is not possible to sufficiently disperse the black magnetic acicular composite particles in the magnetic coating composition. As a result, when a coating film is produced from such a magnetic coating composition, it is difficult to obtain a coating film having a sufficiently smooth surface. Further, since the black magnetic acicular composite particles cannot be sufficiently bound with each other by the binder resin, the obtained coating film tends to become brittle.

In the magnetic recording medium according to the present invention, the amount of carbon black fine particles added to the magnetic recording layer thereof can be reduced to usually less than 6 parts by weight, preferably less than 5 parts by weight, more preferably less than 3 parts by weight based on 100 parts by weight of the black magnetic acicular composite particles.

Further, in the case where the black magnetic acicular composite particles wherein a large amount of the carbon black can be coated onto the surface thereof, especially in an amount of 7 to 10 parts by weight based on 100 parts by weight of the magnetic acicular particles, are used as magnetic particles, it can be expected to omit the addition of the carbon black fine particles to the magnetic recording layer.

Incidentally, the magnetic recording layer may optionally contain a lubricant, an abrasive, an anti-static agent and other additives which are usually used for the production of magnetic recording media, in an amount of 0.1 to 50 parts by weight based on 100 parts of the binder resin.

The magnetic recording medium of the fifth to eighth aspects in the present invention, comprises a non-magnetic base film, a non-magnetic undercoat layer formed on the non-magnetic base film comprising a binder resin and non-magnetic particles, and a magnetic recording layer formed on the non-magnetic undercoat layer, comprising a binder resin and black magnetic acicular composite particles.

The thickness of the non-magnetic undercoat layer is preferably 0.2 to 10.0 μm. When the thickness of the non-magnetic undercoat layer is less than 0.2 μm, it may be difficult to improve the surface roughness of the non-magnetic substrate, and the stiffness of a coating film formed thereon tends to be unsatisfactory. In the consideration of reduction in total thickness of the magnetic recording medium as well as the stiffness of the coating film, the thickness of the non-magnetic undercoat layer is more preferably in the range of 0.5 to 5.0 μm.

As the binder resin, the same binder resin as that used for the production of the magnetic recording layer is usable.

The mixing ratio of the non-magnetic particles to the binder resin is usually 5 to 2000 parts by weight, preferably 100 to 1000 parts by weight based on 100 parts by weight of the binder resin.

When the content of the non-magnetic particles is as small as less than 5 parts by weight, such a non-magnetic undercoat layer in which the non-magnetic particles are uniformly and continuously dispersed may not be obtained upon coating, resulting in insufficient surface smoothness and insufficient stiffness of the non-magnetic substrate. When the content of the non-magnetic particles is more than 2,000 parts by weight, the non-magnetic particles may not be sufficiently dispersed in a non-magnetic coating composition since the amount of the non-magnetic particles is too large as compared to that of the binder resin. As a result, when such a non-magnetic coating composition is coated onto the non-magnetic base film, it may become difficult to obtain a coating film having a sufficiently smooth surface. Further, since the non-magnetic particles may not be sufficiently bonded together through the binder resin, the obtained coating film tends to become brittle.

It is possible to add an additive such as a lubricant, a polishing agent, an antistatic agent, etc. which are generally used for the production of a magnetic recording medium, to the non-magnetic undercoating layer. The mixing ratio of the additive to the binder resin is preferably 0.1 to 50 parts by weight based on 100 parts by weight of the binder resin.

As the non-magnetic particles used in the non-magnetic undercoat layer of the present invention, there may be exemplified non-magnetic inorganic particles ordinarily used for forming a non-magnetic undercoat layer in conventional magnetic recording media. Specific examples of the non-magnetic particles may include hematite particles, iron oxide hydroxide particles, titanium oxide particles, zinc oxide particles, tin oxide particles, tungsten oxide particles, silicon dioxide particles, α-alumina particles, β-alumina particles, γ-alumina particles, chromium oxide particles, cerium oxide particles, silicon carbide particles, titanium carbide particles, silicon nitride particles, boron nitride particles, calcium carbonate particles, barium carbonate particles, magnesium carbonate particles, strontium carbonate particles, calcium sulfate particles, barium sulfate particles, molybdenum disulfide particles, barium titanate particles or the like. These non-magnetic particles may be used singly or in the form of a mixture of any two or more thereof. Among them, the use of hematite particles, iron oxide hydroxide particles, titanium oxide particles and the like is preferred.

In the present invention, in order to improve the dispersibility of the non-magnetic particles in vehicle upon the production of non-magnetic coating composition, the non-magnetic particles may be surface-treated with hydroxides of aluminum, oxides of aluminum, hydroxides of silicon, oxides of silicon or the like to form a coat made of any of these compounds on the surfaces thereof. Further, the non-magnetic particles may contain Al, Ti, Zr, Mn, Sn, Sb or the like inside thereof, if required, in order to improve various properties of the obtained magnetic recording media such as light transmittance, surface electrical resistivity, mechanical strength, surface smoothness, durability or the like.

The particle shape of the hematite particles as the core particles may include a granular shape such as a spherical shape, an irregular (anisotropic) shape, an octahedral shape, a hexahedral shape, a polyhedral shape or the like; an acicular shape such as a needle shape, a spindle shape, a rice ball shape or the like; and a plate shape, or the like.

The lower limit of the average particle size of the hematite particles as the core particles is usually 0.075 μm, preferably 0.085 μm, more preferably 0.095 μm, and the upper limit thereof is usually 0.95 μm, preferably 0.65 μm, more preferably 0.45 μm.

(i) In the case where the shape of the core particles is granular-shaped, the lower limit of the average particle diameter of the granular-shaped hematite particles is usually 0.075 μm, preferably 0.085 μm, more preferably 0.095 μm, and the upper limit thereof is usually 0.95 μm, preferably 0.65 μm, more preferably 0.45 μm.

(ii) In the case where the shape of the core particles is acicular-shaped, the lower limit of the average particle diameter (average major axis diameter) of the acicular-shaped hematite particles is usually 0.075 μm, preferably 0.085 μm, more preferably 0.095 μm, and the upper limit thereof is usually 0.95 μm, preferably 0.65 μm, more preferably 0.45 μm; and-the lower limit of the aspect ratio (average major axis diameter/average minor axis diameter) of the acicular-shaped hematite particles is usually 2:1, preferably 2.5:1, more preferably 3:1, and the upper limit thereof is usually 20:1, preferably 15:1, more preferably 10:1.

(iii) In the case where the shape of the core particles is plate-shaped, the lower limit of the average particle diameter (average plate surface diameter) of the plate-shaped hematite particles is usually 0.075 μm, preferably 0.085 μm, more preferably 0.095 μm, and the upper limit thereof is usually 0.95 μm, preferably 0.65 μm, more preferably 0.45 μm; and the lower limit of the plate ratio (average plate surface diameter/average thickness) of the plate-shaped hematite particles is usually 2:1, preferably 2.5:1, more preferably 3:1, and the upper limit thereof is usually 50:1, preferably 45:1, more preferably 40:1.

The magnetic recording medium according to the present invention has a coercive force of usually 500 to 3500 Oe (39.8 to 278.5 kA/m); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95; a gloss (of the coating film) of usually 165 to 300%; a surface roughness Ra (of the coating film) of usually not more than 11.5 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160; a linear adsorption coefficient (of the coating film) of usually 1.75 to 10.00 μm⁻¹; a surface electrical resistivity of usually not more than 1.0×10⁸ Ω/cm²; a running durability time thereof of usually not less than 24 minutes.

The properties of the magnetic recording media of the first and second aspects are generally comprise: a non-magnetic base film and a magnetic recording layer formed on the non-magnetic base film.

In case of using the black magnetic acicular composite particles, wherein the magnetic acicular cobalt-coated iron oxide particles are used as core particles, which are coated with no hydroxides and/or oxides of aluminum and/or silicon, the magnetic recording medium of the first aspect in the present invention has a coercive force of usually 500 to 1,700 Oe (39.8 to i35.3 kA/m), preferably 550 to 1,700 Oe (43.8 to 135.3 kA/m); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss (of the coating film) of usually 165 to 300%, preferably 170 to 300%; a surface roughness Ra (of the coating film) of usually not more than 11.5 nm, preferably 2.0 to 10.5 rim, more preferably 2.0 to 9.5 nm, a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 125 to 160; a linear adsorption coefficient (of the coating film) of usually 1.75 to 10.0 μm⁻¹, preferably 1.80 to 10.0 μm⁻¹; and a surface electrical resistivity of usually not more than 1.0×10⁸ Ω/cm², preferably not more than 7.5×107 Ω/cm², more preferably not more than 5.0×10⁷ Ω/cm²; a running durability time thereof of usually not less than 24 minutes, preferably not less than 25 minutes; a friction coefficient of usually 0.25 to 0.30, preferably 0.25 to 0.29.

In case of using the black magnetic acicular composite particles, wherein the magnetic acicular metal particles containing iron as a main component are used as core particles which are coated with no hydroxides and/or oxides of aluminum and/or silicon, the magnetic recording medium of the first aspect in the present invention has a coercive force of usually 800 to 3,500 Oe (63.7 to 278.5 kA/m), preferably 900 to 3,500 Oe (71.6 to 278.5 kA/m); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss (of the coating film) of usually 185 to 300%, preferably 195 to 300%; a surface roughness Ra (of the coating film) of usually not more than 11.5 nm, preferably 2.0 to 10.5 nm, more preferably 2.0 to 9.5 nm, a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 125 to 160; a linear adsorption coefficient (of the coating film) of usually 1.85 to 10.0 μm⁻¹, preferably 1.90 to 10.0 μm⁻¹; and a surface electrical resistivity of usually not more than 1.0×10⁸ Ω/cm², preferably not more than 7.5×10⁷ Ω/cm², more preferably not more than 5.0×10⁷ Ω/cm²; a running durability time thereof of usually not less than 24 minutes, preferably not less than 25 minutes; a friction coefficient of usually 0.25 to 0.30, preferably 0.25 to 0.29.

In case of using the black magnetic acicular composite particles, wherein the magnetic acicular cobalt-coated iron oxide particles as core particles which are coated with hydroxides and/or oxides of aluminum and/or silicon, the magnetic recording medium of the second aspect in the present invention has a coercive force of usually 500 to 1,700 Oe (39.8 to 135.3 kA/m), preferably 550 to 1,700 Oe (43.8 to 135.3 kA/m); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss (of the coating film) of usually 170 to 300%, preferably 175 to 300%; a surface roughness Ra (of the coating film) of usually not more than 10.5 nm, preferably 2.0 to 9.5 nm, more preferably 2.0 to 8.5 nm, a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 127 to 160; a linear adsorption coefficient (of the coating film) of usually 1.75 to 10.0 μm⁻¹, preferably 1.80 to 10.0 μm⁻¹; and a surface electrical resistivity of usually not more than 1.0×10⁸ Ω/cm², preferably not more than 7.5×10⁷ Ω/cm², more preferably not more than 5.0×10⁷ Ω/cm²; a running durability time thereof of usually not less than 26 minutes, preferably not less than 27 minutes; a friction coefficient of usually 0.21 to 0.26, preferably 0.21 to 0.25.

In case of using the black magnetic acicular composite particles, wherein the magnetic acicular metal particles containing iron as a main component as core particles which are coated with hydroxides and/or oxides of aluminum and/or silicon, the magnetic recording medium the second aspect in the present invention has a coercive force of usually 800 to 3500 Oe (63.7 to 278.5 kA/m), preferably 900 to 3500 Oe (71.6 to 278.5 kA/m); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss (of the coating film) of usually 190 to 300%, preferably 200 to 300%; a surface roughness Ra (of the coating film) of usually not more than 10.5 nm, preferably 2.0 to 9.5 nm, more preferably 2.0 to 8.5 nm, a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 127 to 160; a linear adsorption coefficient (of the coating film) of usually 1.85 to 10.0 μm⁻¹, preferably 1.90 to 10.0 μm⁻¹; and a surface electrical resistivity of usually not more than 1.0×10⁸ Ω/cm², preferably not more than 7.5×10⁷ Ω/cm², more preferably not more than 5.0×10⁷ Ω/cm²; a running durability time thereof of usually not less than 26 minutes, preferably not less than 27 minutes; a friction coefficient of usually 0.21 to 0.26, preferably 0.21 to 0.25.

The properties of the magnetic recording media of the third to sixth aspect are generally comprise: a non-magnetic base film, a non-magnetic undercoat layer formed on the non-magnetic base film, and a magnetic recording layer formed on the non-magnetic undercoat layer.

In case of using the black magnetic acicular composite particles, wherein the magnetic acicular cobalt-coated iron oxide particles which are coated with no hydroxides and/or oxides of aluminum and/or silicon, the magnetic recording medium the fifth aspect the present invention has a coercive force of usually 500 to 1,700 Oe (39.8 to 135.3 kA/m), preferably 550 to 1,700 Oe (43.8 to 135.3 kA/m); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss (of the coating film) of usually 165 to 300%, preferably 170 to 300%; a surface roughness Ra (of the coating film) of usually not more than 11.0 nm, preferably 2.0 to 10.5 nm, more preferably 2.0 to 10.0 nm, a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 125 to 160, preferably 126 to 160; a linear adsorption coefficient (of the coating film) of usually 1.40 to 10.0 μm⁻¹, preferably 1.45 to 10.0 μm⁻¹; and a surface electrical resistivity of usually not more than 1.0×10⁹ Ω/cm², preferably not more than 7.5×10⁸ Ω/cm², more preferably not more than 5.0×10⁸ Ω/cm²; a running durability time thereof of usually not less than 25 minutes, preferably not less than 26 minutes.

In case of using the black magnetic acicular composite particles wherein the magnetic acicular cobalt-coated iron oxide particles which are coated with no hydroxides and/or oxides of aluminum and/or silicon, the magnetic recording medium of the third aspect not only can satisfy the above described properties, but also can exhibit a linear absorption (of coating film) of usually 1.80 to 10.00 μm⁻¹, preferably 1.85 to 10.00 μm⁻¹; a surface electrical resistivity value (of coating film) of usually not more than 1.0×10⁸ Ω/cm², preferably not more than 7.5×10⁷ Ω/cm², more preferably not more than 5.0×10⁷ Ω/cm²; a friction coefficient of usually 0.25 to 0.30, preferably 0.25 to 0.29; and a running durability time thereof of usually not less than 26 minutes, preferably not less than 27 minutes.

In case of using the black magnetic acicular composite particles, wherein the magnetic acicular metal particles containing iron as a main component which are coated with no hydroxides and/or oxides of aluminum and/or silicon, the magnetic recording medium the fifth aspect in the present invention has a coercive force of usually 800 to 3,500 Oe (63.7 to 278.5 kA/m), preferably 900 to 3,500 Oe (71.6 to 278.5 kA/m); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss (of the coating film) of usually 185 to 300%, preferably 195 to 300%; a surface roughness Ra (of the coating film) of usually not more than 11.0 nm, preferably 2.0 to 10.5 nm, more preferably 2.0 to 10.0 nm, a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 125 to 160, preferably 126 to 160; a linear adsorption coefficient (of the coating film) of usually 1.40 to 10.0 μm-1, preferably 1.45 to 10.0 μm⁻¹; and a surface electrical resistivity of usually not more than 1.0×10⁹ Ω/cm², preferably not more than 7.5×10⁸ Ω/cm², more preferably not more than 5.0×10⁸ Ω/cm²; a running durability time thereof of usually not less than 25 minutes, preferably not less than 26 minutes.

In case of using the black magnetic acicular composite particles, wherein the magnetic acicular metal particles containing iron as a main component which are coated with hydroxides and/or oxides of aluminum and/or silicon, the magnetic recording medium of the third aspect not only can satisfy the above described properties, but also can exhibit a linear absorption (of coating film) of usually 1.80 to 10.00 μm⁻¹, preferably 1.85 to 10.00 μm⁻¹; a surface electrical resistivity value (of coating film) of usually not more than 1.0×10⁸ Ω/cm², preferably not more than 7.5×10⁷ Ω/cm², more preferably not more than 5.0×10⁷ Ω/cm²; a friction coefficient of usually 0.25 to 0.30, preferably 0.25 to 0.29; and a running durability time thereof of usually not less than 26 minutes, preferably not less than 27 minutes.

In case of using the black magnetic acicular composite particles, wherein the magnetic acicular cobalt-coated iron oxide particles which are coated with hydroxides and/or oxides of aluminum and/or silicon, the magnetic recording medium of the sixth aspect in the present invention has a coercive force of usually 500 to 1,700 Oe (39.8 to 135.3 kA/m), preferably 550 to 1,700 Oe (43.8 to 135.3 kA/m); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss (of the coating film) of usually 170 to 300%, preferably 175 to 300%; a surface roughness Ra (of the coating film) of usually not more than 10.0 nm, preferably 2.0 to 9.5 nm, more preferably 2.0 to 9.0 nm, a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 127 to 160, preferably 128 to 160; a linear adsorption coefficient (of the coating film) of usually 1.45 to 10.0 μm⁻¹, preferably 1.50 to 10.0 μm⁻¹; and a surface electrical resistivity of usually not more than 1.0×10⁹ Ω/cm², preferably not more than 7.5×10⁸ Ω/cm², more preferably not more than 5.0×10⁸ Ω/cm²; a running durability time thereof of usually not less than 26 minutes, preferably not less than 27 minutes.

In case of using the black magnetic acicular composite particles, wherein the magnetic acicular cobalt-coated iron oxide particles which are coated with hydroxides and/or oxides of aluminum and/or silicon, the magnetic recording medium of the fourth aspect not only can satisfy the above described properties, but also can exhibit a linear absorption (of coating film) of usually 1.85 to 10.00 μm⁻¹, preferably 1.90 to 10.00 μm⁻¹; a surface electrical resistivity value (of coating film) of usually not more than 5.0×10⁷ Ω/cm², preferably not more than 2.5×10⁷ Ω/cm², more preferably not more than 1.0×10⁷ Ω/cm²; a friction coefficient of usually 0.21 to 0.26, preferably 0.21 to 0.25; and a running durability time thereof of usually not less than 27 minutes, preferably not less than 28 minutes.

In case of using the black magnetic acicular composite particles, wherein the magnetic acicular metal particles containing iron as a main component which are coated with hydroxides and/or oxides of aluminum and/or silicon, the magnetic recording medium the sixth aspect in the present invention has a coercive force of usually 800 to 3,500 Oe (63.7 to 278.5 kA/m), preferably 900 to 3,500 Oe (71.6 to 278.5 kA/m); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss (of the coating film) of usually 190 to 300%, preferably 200 to 300%; a surface roughness Ra (of the coating film) of usually not more than 10.0 nm, preferably 2.0 to 9.5 nm, more preferably 2.0 to 9.0 nm, a Young's modulus (relative value to a commercially available videotape: AV T-120 produced by Victor Company of Japan, Limited) of usually 127 to 160, preferably 128 to 160; a linear adsorption coefficient (of the coating film) of usually 1.45 to 10.0 μm⁻¹, preferably 1.50 to 10.0 μm⁻¹; and a surface electrical resistivity of usually not more than 1.0×10⁹ Ω/cm², preferably not more than 7.5×10⁸ Ω/cm², more preferably not more than 5.0×10⁸ Ω/cm²; a running durability time thereof of usually not less than 26 minutes, preferably not less than 27 minutes.

In case of using the black magnetic acicular composite particles, wherein the magnetic acicular metal particles containing iron as a main component which are coated with hydroxides and/or oxides of aluminum and/or silicon, the magnetic recording medium of the fourth aspect not only can satisfy the above described properties, but also can exhibit a linear absorption (of coating film) of usually 1.85 to 10.00 μm⁻¹, preferably 1.90 to 10.00 μm⁻¹; a surface electrical resistivity value (of coating film) of usually not more than 5.0×10⁷ Ω/cm², preferably not more than 2.5×10⁷ Ω/cm², more preferably not more than 1.0×10⁷ Ω/cm²; a friction coefficient of usually 0.21 to 0.26, preferably 0.21 to 0.25; and a running durability time thereof of usually not less than 27 minutes, preferably not less than 28 minutes.

The black magnetic acicular composite particles according to the present invention can be produced by the following method.

As the core particles used in the present invention, known Co-coated magnetic acicular iron oxide particles and magnetic acicular metal particles containing iron as a main component may be used.

The coating of the magnetic acicular particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes, may be conducted (i) by mechanically mixing and stirring the magnetic acicular particles together with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes; or (ii) by mechanically mixing and stirring both the components together while spraying the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes onto the magnetic acicular particles. In these cases, substantially whole amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes added can be applied onto the surfaces of the magnetic acicular particles.

In addition, by conducting the above-mentioned mixing or stirring treatment (i) of the hematite particles as core particles together with the alkoxysilane compounds, at least a part of the alkoxysilane compounds coated on the hematite particles as core particles may be changed to the organosilane compounds. In this case, there is also no affection against the formation of the carbon black coat thereon.

In order to uniformly coat the surfaces of the magnetic acicular particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes, it is preferred that the magnetic acicular particles are preliminarily diaggregated by using a pulverizer.

As apparatus (a) for mixing and stirring treatment (i) of the core particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes to form the coating layer thereof, and as apparatus (b) for mixing and stirring treatment (ii) of carbon black fine particles with the particles whose surfaces are coated with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes to form the carbon black coat, there may be preferably used those apparatus capable of applying a shear force to the particles, more preferably those apparatuses capable of conducting the application of shear force, spaturate force and compressed force at the same time. In addition, by conducting the above mixing or stirring treatment (a) of the core particles together with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes, at least a part of the alkoxysilane compounds coated on the core particles may be changed to the organosilane compounds.

As such apparatuses, there may be exemplified wheel-type kneaders, ball-type kneaders, blade-type kneaders, roll-type kneaders or the like. Among them, wheel-type kneaders are preferred.

Specific examples of the wheel-type kneaders may include an edge runner (equal to a mix muller, a Simpson mill or a sand mill), a multi-mull, a Stotz mill, a wet pan mill, a Conner mill, a ring muller, or the like. Among them, an edge runner, a multi-mull, a Stotz mill, a wet pan mill and a ring muller are preferred, and an edge runner is more preferred.

Specific examples of the ball-type kneaders may include a vibrating mill or the like. Specific examples of the blade-type kneaders may include a Henschel mixer, a planetary mixer, a Nawter mixer or the like. Specific examples of the roll-type kneaders may include an extruder or the like.

In order to coat the surfaces of the core particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes as uniformly as possible, the conditions of the above mixing or stirring treatment may be appropriately controlled such that the linear load is usually 2 to 200 Kg/cm (19.6 to 1960 N/cm), preferably 10 to 150 Kg/cm (98 to 1470 N/cm), more preferably 15 to 100 Kg/cm (147 to 980 N/cm); and the treating time is usually 5 to 120 minutes, preferably 10 to 90 minutes. It is preferred to appropriately adjust the stirring speed in the range of usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.

The amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes added, is preferably 0.15 to 45 parts by weight based on 100 parts by weight of the magnetic acicular particles.

Next, the carbon black fine particles are added to the magnetic acicular particles coated with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes, and the resultant mixture is mixed and stirred to form the carbon black coat on the surfaces of the coating layer composed of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes.

It is preferred that the carbon black fine particles are added little by little and slowly, especially about 5 to 60 minutes.

In order to form carbon black coat onto the coating layer composed of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes as uniformly as possible, the conditions of the above mixing or stirring treatment can be appropriately-controlled such that the linear load is usually 2 to 200 Kg/cm (19.6 to 1960 N/cm), preferably 10 to 150 Kg/cm (98 to 1470 N/cm), more preferably 15 to 100 Kg/cm (147 to 980 N/cm); and the treating time is usually 5 to 120 minutes, preferably 10 to 90 minutes. It is preferred to appropriately adjust the stirring speed in the range of usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.

The amount of carbon black fine particles added is 0.5 to 40 parts by weight based on 100 parts by weight of the core particles. When the amount of carbon black fine particles added is less than 0.5 part by weight, the amount of carbon black coat may become too small. As a result, it may be difficult to obtain black magnetic acicular composite particles having an excellent blackness and a low volume resistivity value. When the amount of carbon black fine particles added is more than 40 parts by weight, the effect of improving the blackness and the volume resistivity value of the obtained black magnetic acicular composite particles is already saturated and, therefore, the addition of such a large amount of carbon black fine particles is unnecessary and meaningless.

In particular, in order to obtain black magnetic acicular composite particles having a carbon black coat in am amount of from more than 10 to 40 parts by weight based on 100 parts by weight of the core particles, it is preferred to add carbon black in two or more separate parts while alternately repeating the addition and adhesion thereof. Especially, when carbon black fine particles for forming the second carbon black coat is added and adhered onto the first carbon black coat, it is preferred to adhere an adhesive onto the first carbon black coat prior to the addition of carbon black fine particles for the second carbon black coat.

More specifically, the adhesive is added to the composite particles on which the first carbon black coat is formed, and the mixture is mixed and stirred together to adhere the adhesive onto the particles. Then, carbon black fine particles are further added to the obtained composite particles, and these particles are mixed and stirred together to form the carbon black coat onto the first carbon black coat, thereby forming the second carbon black coat on the first carbon black coat.

The mixing and stirring conditions with the adhesives may be adequately controlled such that the linear load is usually 2 to 200 Kg/cm (19.6 to 1960 N/cm), preferably 10 to 150 Kg/cm (98 to 1470 N/cm), more preferably 15 to 100 Kg/cm (147 to 980 N/cm); the treating time is usually 5 to 120 minutes, preferably 10 to 90 minutes; and the stirring speed is usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.

The amount of the adhesive added is 0.1 to 5 parts by weight based on 100 parts by weight of the core particles. When the amount of the adhesive added is less than 0.1 part by weight, it may become difficult to form a sufficient amount of carbon black coat on the first carbon coat, thereby failing to form a sufficient second carbon black coat thereon. When the amount of the adhesive added is more than 5 parts by weight, the adhesion effect is already saturated and, therefore, it is unnecessary and meaningless to add such a large amount of the adhesive.

The mixing and stirring conditions upon adhesion of the second carbon black coat may be appropriately selected such that the carbon black can be uniformly adhered to the adhesive. More specifically, the treating conditions may be adequately controlled such that the linear load is usually 2 to 200 Kg/cm (19.6 to 1960 N/cm), preferably 10 to 150 Kg/cm (98 to 1470 N/cm), more preferably 15 to 100 Kg/cm (147 to 980 N/cm); the treating time is 5 to 120 minutes, preferably 10 to 90 minutes; and the stirring speed is usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm.

The amount of carbon black fine particles added for forming the second carbon black coat is 1 to 30 parts by weight based on 100 parts by weight of the core particles. When the amount of the carbon black fine particles added is less than 1 part by weight, the amount of carbon black adhered onto the surface of the adhesive is too small, so that the adhesive may be exposed to the surface of each particle. When the amount of the carbon black fine particles added is more than 30 parts by weight, the obtained black magnetic acicular composite particles tend to suffer from desorption of carbon black from the surfaces thereof, resulting in deteriorated dispersibility in vehicle.

In the present invention, after carbon black is adhered onto the core particles, the obtained black magnetic acicular composite particles may be dried or heat-treated. The temperature used in the drying or heat-treatment, is preferably 40 to 200° C., more preferably 60 to 150° C., and the heating time is preferably from 10 minutes to 12 hours, more preferably from 30 minutes to 3 hours. By conducting such drying or heat-treatment, the alkoxysilane can be converted into organosilane compounds.

The alkoxysilane used to coat the core particles in the thus obtained composite particles is finally converted into the organosilane compound through the these steps.

It is preferred that the cobalt-coated magnetite particles, the magnetic acicular metal particles containing iron as a main component, the magnetic acicular iron alloy particles or the like, which are susceptible to oxidation, be placed within a mixing device and purged with an inert gas such as N₂ gas in order to prevent the deterioration in magnetic properties thereof by the oxidation.

At least a part of the surface of the magnetic acicular particles may be coated with at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon, if required, in advance of mixing and stirring with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes, or the terminal-modified polysiloxanes.

The coating of the hydroxides and/or oxides of aluminum and/or silicon may be conducted by adding an aluminum compound, a silicon compound or both the compounds to a water suspension in which the magnetic acicular particles are dispersed, followed by mixing and stirring, and further adjusting the pH value of the suspension, if required, thereby coating the surfaces of the magnetic acicular particles with at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon. The thus obtained magnetic acicular particles coated with the hydroxides and/or oxides of aluminum and/or silicon are then filtered out, washed with water, dried and pulverized. Further, the particles coated with the hydroxides and/or oxides of aluminum and/or silicon may be subjected to post-treatments such as deaeration treatment and compaction treatment, if required.

As the aluminum compounds, there may be exemplified aluminum salts such as aluminum acetate, aluminum sulfate, aluminum chloride or aluminum nitrate, alkali aluminates such as sodium aluminate or the like.

The amount of the aluminum compound added is 0.01 to 20% by weight (calculated as Al) based on the weight of the magnetic acicular particles. When the amount of the aluminum compound added is less than 0.01% by weight, it may be difficult to sufficiently coat the surfaces of the magnetic acicular particles with hydroxides and/or oxides of aluminum, thereby failing to improve the effective reduction of the carbon black desorption percentage. On the other hand, when the amount of the aluminum compound added is more than 20% by weight, the coating effect is saturated and, therefore, it is meaningless to add such an excess amount of the aluminum compound.

As the silicon compounds, there may be exemplified #3 water glass, sodium orthosilicate, sodium metasilicate or the like.

The amount of the silicon compound added is 0.01 to 20% by weight (calculated as SiO₂) based on the weight of the magnetic acicular particles. When the amount of the silicon compound added is less than 0.01% by weight, it may be difficult to sufficiently coat the surfaces of the magnetic acicular particles with hydroxides and/or oxides of silicon, thereby failing to improve the effective reduction of the carbon black desorption percentage. On the other hand, when the amount of the silicon compound added is more than 20% by weight, the coating effect is saturated and, therefore, it is meaningless to add such an excess amount of the silicon compound.

In the case where both the aluminum and silicon compounds are used in combination for the coating, the total amount of the aluminum and silicon compounds added is preferably 0.01 to 20% by weight (calculated as a sum of Al and SiO₂) based on the weight of the magnetic acicular particles.

Next, the process for producing the magnetic recording medium according to the present invention is described.

The magnetic recording medium according to the first and second aspects of the present invention can be produced by an ordinary method, i.e., by coating the surface of the non-magnetic base film with a magnetic coating composition comprising black magnetic acicular composite particles, a binder resin, and a solvent to form a magnetic recording layer thereon, and then magnetically orienting the magnetic recording layer.

The magnetic recording medium according to the third to sixth aspects of the present invention can be produced by an ordinary method, i.e., by coating the surface of the non-magnetic base film with a non-magnetic coating composition comprising non-magnetic particles, a binder resin and a solvent to form a coating film thereon; drying the coating film to form a non-magnetic undercoat layer; coating the surface of the non-magnetic undercoat layer with a magnetic coating composition comprising black magnetic acicular composite particles, a binder resin and a solvent to form a magnetic recording layer thereon, and then magnetically orienting the magnetic recording layer.

Upon kneading and dispersing the non-magnetic coating composition and magnetic coating composition, as kneaders, there may be used, for example, twin-screw kneader, twin-screw extruder, pressure kneader, twin-roll mill, triple-roll mill or the like; and as dispersing devices, there may be used ball mill, sand grinder, attritor, disper, homogenizer, ultrasonic dispersing device or the like.

The coating of the non-magnetic coating composition and magnetic coating composition may be conducted using gravure coater, reverse-roll coater, slit coater, die coater or the like. The thus obtained coating film may be magnetically oriented using counter magnet, solenoid magnet or the like.

As the solvents, there may be used methyl ethyl ketone, toluene, cyclohexanone, methyl isobutyl ketone, tetrahydrofuran, a mixture of these solvents or the like.

The total amount of the solvent used is 65 to 1,000 parts by weight based on 100 parts by weight of the black magnetic acicular composite particles. When the amount of the solvent used is less than 65 parts by weight, the viscosity of the magnetic coating composition prepared therefrom becomes too high, thereby making it difficult to apply the magnetic coating composition. On the other hand, when the amount of the solvent used is more than 1,000 parts by weight, the amount of the solvent volatilized during the formation of the coating film becomes too large, thereby rendering the coating process industrially disadvantageous.

The one point of the present invention is the fact that when carbon black coatis firmly formed onto the surfaces of the core particles in an amount as large as from more than 10 to 40 parts by weight based on 100 parts by weight of the core particles, the obtained magnetic acicular composite particles can exhibit a well-controlled myristic acid-adsorption of 0.01 to 0.3 mg/m².

The reason why the magnetic acicular composite particles of the present invention can exhibit a reduced myristic acid-adsorption, is considered as follows, though not clearly known yet. That is, due to the fact that the myristic acid-adsorption of the magnetic acicular particles cannot be reduced to not more than 0.3 mg/m² in any of the cases where the carbon black desorption percentage is more than 20% even though the amount of carbon black adhered is more than 10 parts by weight, and where the amount of carbon black adhered is not more than 10 parts by weight even though the carbon black desorption percentage is not more than 20%, as described in Comparative Examples below, it is considered that many hydroxyl groups present on the surface of the core particles are prevented from being bonded with carboxyl groups of myristic acid which show a high affinity to the hydroxyl groups, by the synergistic effect of the dense carbon black coat formed on the surfaces of the core particles which has an adequate thickness, and dimethyl polysiloxane as adhesive.

Also, the magnetic recording medium of the first and second aspect of the present invention using the above magnetic acicular particles according to the present invention can exhibit a low friction coefficient and an excellent running durability.

The reason why the magnetic recording medium of the first and second aspect of the present invention can show a reduced friction coefficient, is considered by the present inventors as follows. That is, since the amount of myristic acid adsorbed onto the surfaces of the magnetic acicular particles which are incorporated in a large amount into a magnetic recording layer thereof, is restricted to the specified range, an appropriate amount of myristic acid can be oozed onto the surface of the magnetic recording layer in a well-controlled condition for a long period of time, thereby effectively exhibiting a lubricating function thereof.

The reason why the magnetic recording medium of the first and second aspect of the present invention can show an excellent running durability, is considered by the present inventors as follows. That is, for the same reason as described above, an appropriate amount of myristic acid can be oozed onto the surface of the magnetic recording layer in a well-controlled condition, resulting in stable running durability of the magnetic recording medium.

Another point of the present invention is the fact that the magnetic recording medium of the third to sixth aspects which has a non-magnetic undercoat layer and is obtained by using the black magnetic acicular composite particles as magnetic particles, can exhibit a small light transmittance and a low surface electrical resistivity value even though the amount of carbon black incorporated in a magnetic recording layer thereof is reduced to as small a level as possible, and can be enhanced in surface smoothness of the magnetic recording layer.

The reason why the magnetic recording medium of the third to sixth aspects of the present invention can show a small light transmittance irrespective of a less amount of carbon black added thereto, is considered by the present inventors as follows. That is, carbon black fine particles usually tend to form agglomerates due to fineness thereof. However, in the black magnetic acicular composite particles of the present invention, since the carbon black fine particles can be uniformly and densely adhered onto the surfaces of the core particles, the individual carbon black fine particles can exhibit a function thereof more effectively.

The reason why the magnetic recording medium of the third to sixth aspects of the present invention can show a low surface electrical resistivity value irrespective of a less amount of carbon black added thereto, is considered by the present inventors as follows. That is, since the black magnetic acicular composite particles are uniformly dispersed in the coating film, the carbon black fine particles uniformly and densely adhered onto the surfaces of the respective composite particles are continuously connected and contacted with each other in the coating film.

The reason why the magnetic recording medium of the third to sixth aspects of the present invention can show an excellent surface smoothness, is considered by the present inventors as follows. That is, in the black magnetic acicular composite particles of the present invention, the amount of carbon black fine particles desorbed or fallen-off from the surfaces of the particles is very small, and the amount of carbon black fine particles added into the magnetic recording layer of the magnetic recording medium can be reduced to as small a level as possible. For these reasons, the black magnetic acicular composite particles are prevented from being deteriorated in dispersibility in vehicle due to desorbed and liberated carbon black fine particles upon the production of magnetic coating composition. Further, the black magnetic acicular composite particles themselves are excellent in dispersibility in vehicle.

The magnetic recording medium of the third and fourth aspects according to the present invention in which the black magnetic acicular composite particles having a well-controlled myristic acid-adsorption are used, can exhibit a low friction coefficient and an excellent running durability.

The reason why the magnetic recording medium of the third and fourth aspects according to the present invention can be reduced in friction coefficient, is considered by the present inventors as follows. That is, by controlling the amount of myristic acid adsorbed onto the surfaces of the black magnetic acicular composite particles which are incorporated in a large amount into a magnetic recording layer of the magnetic recording medium, to the specified range, an appropriate amount of myristic acid can be oozed onto the surface of the magnetic recording layer in a well-controlled condition for a long period of time, thereby effectively exhibiting a good lubricating function thereof.

The reason why the magnetic recording medium of the third and fourth aspects of the present invention has an excellent running durability, is considered by the present inventors as follows. That is, for the same reason as described above, an appropriate amount of myristic acid can be oozed onto the surface of the magnetic recording layer in a well-controlled condition, resulting in stable running durability of the magnetic recording medium.

Further, the black magnetic acicular composite particles of the present invention can exhibit an excellent dispersibility in vehicle, a more excellent blackness, a lower volume resistivity value and a well-controlled myristic acid-adsorption. Therefore, when the particles are used for the production of magnetic recording medium, the obtained magnetic recording medium can exhibits a smooth surface, a more excellent blackness and a lower surface electrical resistivity value, and can be well-controlled in amount of myristic acid oozed onto the surface of a magnetic recording layer. Accordingly, the magnetic acicular composite particles of the present invention is suitable as magnetic acicular particles for magnetic recording media.

By using the above black magnetic acicular composite particles, the magnetic recording medium according to the present invention can exhibit not only a smooth surface, a smaller light transmittance and a lower surface electrical resistivity value, but also a small friction coefficient and an excellent running durability. Therefore, the magnetic recording medium of the present invention is suitable as those for high-density recording.

Also, in the magnetic recording medium of the third and fourth aspects of the present invention, the non-magnetic undercoat layer is formed, and the black magnetic acicular composite particles having an excellent blackness and a low volume resistivity value are used as magnetic particles therefor. Therefore, the obtained magnetic recording medium can exhibit a small light transmittance and a low surface electrical resistivity value. Further, since the amount of carbon black fine particles incorporated into the magnetic recording layer is reduced to as small a level as possible and the black magnetic acicular composite particles themselves are enhanced in dispersibility in vehicle, the obtained magnetic recording medium can exhibit a smooth surface. Accordingly, the magnetic recording medium of the present invention is suitable as those for high-density recording.

Further, the black magnetic acicular composite particles adhered with a large amount of carbon black, exhibit a well-controlled myristic acid-adsorption. Therefore, the magnetic recording medium of the present invention using the above black magnetic acicular composite particles as magnetic particles, can exhibit a small friction coefficient and an excellent running durability in addition to the above-described properties. Accordingly, the magnetic recording medium of the present invention is suitable as those for high-density recording.

EXAMPLES

The present invention is described in more detail by Examples and Comparative Examples, but the Examples are only illustrative and, therefore, not intended to limit the scope of the present invention.

Various properties were measured by the following methods.

(1) The average particle diameter, the average major axial diameter and average minor axial diameter of magnetic acicular particles, composite particles, black magnetic acicular composite particles and carbon black fine particles were respectively expressed by the average of values (measured in a predetermined direction) of about 350 particles which were sampled from a micrograph obtained by magnifying an original electron micrograph by four times in each of the longitudinal and transverse directions.

(2) The aspect ratio of the particles was expressed by the ratio of average major axial diameter to average minor axial diameter thereof.

(3) The geometrical standard deviation of particle diameter was expressed by values obtained by the following method. That is, the particle sizes were measured from the above magnified electron micrograph. The actual particle sizes and the number of the particles were calculated from the measured values. On a logarithmic normal probability paper, the particle sizes were plotted at regular intervals on the abscissa-axis and the accumulative number (under integration sieve) of particles belonging to each interval of the particle sizes were plotted by percentage on the ordinate-axis by a statistical technique.

The particle sizes corresponding to the number of particles of 50% and 84.13%, respectively, were read from the graph, and the geometrical standard deviation was calculated from the following formula: Geometrical standard deviation={particle sizes corresponding to 84.13% under integration sieve}/{particle sizes (geometrical average diameter) corresponding to 50% under integration sieve}

The closer to 1 the geometrical standard deviation value, the more excellent the particle size distribution.

(4) The specific surface area was expressed by the value measured by a BET method.

(5) The amount of Al, Si and Co which were present within magnetic acicular particles, composite particles or black magnetic acicular composite particles, or on surfaces thereof, and the amount of Si contained in the organosilicon compounds, were measured by a fluorescent X-ray spectroscopy device 3063 M (manufactured by Rigaku Denki Kogyo Co., Ltd.) according to JIS K0119 “General rule of fluorescent X-ray analysis”.

(6) The content of Fe²⁺ in the core particles is expressed by the value measured by the following chemical analysis method.

That is, 25 ml of a mixed solution composed of phosphoric acid and sulfuric acid at a mixing ratio of 2:1, was added to 0.5 g of core particles, thereby dissolving the core particles in the mixed solution. After several droplets of diphenylamine sulfonic acid as an indicator was added to the diluted solution, the solution was subjected to oxidation-reduction titration using an aqueous potassium bichromate solution. The titration was terminated when the diluted solution exhibited a violet color. The amount of Fe²⁺ was measured from the amount of the aqueous potassium bichromate solution used up to the termination of the titration.

(7) The amount of carbon black coat formed on the surface of the composite particles or black magnetic acicular composite particles was measured by “Horiba Metal, Carbon and Sulfur Analyzer EMIA-2200 Model” (manufactured by Horiba Seisakusho Co., Ltd.).

(8) The blackness of the magnetic acicular particles, composite particles and black filler was measured by the following method. That is, 0.5 g of sample particles and 1.5 ml of castor oil were intimately kneaded together by a Hoover's muller to form a paste. 4.5 g of clear lacquer was added to the obtained paste and was intimately kneaded to form a paint. The obtained paint was applied on a cast-coated paper by using a 6-mil (150 μm) applicator to produce a coating film piece (having a film thickness of about 30 μm). The thus obtained coating film piece was measured according to JIS Z 8729 by a multi-light source spectrographic calorimeter MSC-IS-2D (manufactured by Suga Testing Machines Manufacturing Co., Ltd.) to determine an L* value of calorimetric indices thereof. The blackness was expressed by the L* value measured.

Here, the L* value represents a lightness, and the smaller the L* value, the more excellent the blackness.

(9) The volume resistivity of the magnetic acicular particles, composite particles and the black magnetic acicular composite particles was measured by the following method.

That is, first, 0.5 g of the sample particles to be measured was weighted, and press-molded at 140 Kg/cm² (13,720 kPa) using a KBr tablet machine (manufactured by Simazu Seisakusho Co., Ltd.), thereby forming a cylindrical test piece.

Next, the thus obtained cylindrical test piece was exposed to an atmosphere maintained at a temperature of 25° C. and a relative humidity of 60% for 12 hours. Thereafter, the cylindrical test piece was set between stainless steel electrodes, and a voltage of 15V was applied between the electrodes using a Wheatstone bridge (model 4329A, manufactured by Yokogawa-Hokushin Denki Co., Ltd.) to measure a resistance value R (Ω).

The cylindrical test piece was measured with respect to an upper surface area A (cm²) and a thickness t₀ (cm) thereof. The measured values were inserted into the following formula, thereby obtaining a volume resistivity X (Ω·cm). X(Ω·cm)=R×(A/t ₀)

(10) The thickness of carbon black coat formed on the surfaces of the black magnetic acicular composite particles is expressed by the value which was obtained by first measuring an average thickness of carbon black coat formed onto the surfaces of the particles on a photograph (×5,000,000) obtained by magnifying (ten times) a micrograph (×500,000) produced at an accelerating voltage of 200 kV using a transmission-type electron microscope (JEM-2010, manufactured by Japan Electron Co., Ltd.), and then calculating an actual thickness of carbon black coat formed from the measured average thickness.

(11) The desorption Percentage (T %) of carbon black desorbed from the composite particles or black magnetic acicular composite particles was measured by the following method.

That is, 3 g of the sample particles and 40 ml of ethanol were placed in a 50-ml precipitation pipe and then was subjected to ultrasonic dispersion for 20 minutes. Thereafter, the obtained dispersion was allowed to stand for 120 minutes, and separated the carbon black desorbed from the sample particles on the basis of the difference in specific gravity therebetween. Next, the thus separated sample particles were mixed again with 40 ml of ethanol, and the obtained mixture was further subjected to ultrasonic dispersion for 20 minutes. Thereafter, the obtained dispersion was allowed to stand for 120 minutes, thereby separating the sample particles and carbon black desorbed, from each other. The thus separated black magnetic acicular composite particles were dried at 100° C. for one hour, and then the carbon content thereof was measured by the “Horiba Metal, Carbon and Sulfur Analyzer EMIA-2200 Model” (manufactured by HORIBA SEISAKUSHO CO., LTD.). The desorption percentage (T %) was calculated according to the following formula: T(%)={(W _(a) −W _(e))/W _(a)}×100 wherein W_(a) represents an amount of carbon black initially formed on the sample particles; and W_(e) represents an amount of carbon black which still remains on the sample particles after the above desorption test.

The closer to zero the desorption percentage (T %), the smaller the amount of carbon black desorbed from the sample particles.

(12) The myristic acid absorption of the magnetic acicular particles, the composite particles and the black magnetic acicular composite particles was measured by the following method. The lower the myristic acid absorption, the more the fatty acid becomes to ooze on the surface of the magnetic recording layer and the lower the friction coefficient thereof.

100 g of 1.5 mmφ glass beads, 9 g of sample particles to be measured and 45 ml of a tetrahydrofuran solution containing myristic acid in an amount enough to form one layer thereof on each sample particle, were charged into a 140-ml glass bottle, and then mixed and dispersed together for 60 minutes using a paint shaker.

Next, the thus obtained dispersion was taken out, charged into a 50-ml precipitation tube and centrifuged at 10,000 rpm for 15 minutes, thereby separating a solvent portion from a solid portion. The amount (concentration) of myristic acid contained in the solvent portion was determined by a gravimetric method. By subtracting the measured value from an amount of myristic acid initially charged, the amount of myristic acid contained in the solid portion was obtained as a myristic acid absorption (mg/m²) of the particles to be measured.

(13) The viscosity of the coating composition was obtained by measuring the viscosity of the coating composition at 25° C. at a shear rate D of 1.92 sec⁻¹ by using “E type viscometer EMD-R” (manufactured by Tokyo Keiki, Co., Ltd.).

(14) The gloss of the surface of the coating film of the magnetic recording layer was measured at an angle of incidence of 45° by “glossmeter UGV-5D” (manufactured by Suga Shikenki, Co., Ltd.).

(15) The surface roughness Ra is expressed by the average value of the center-line average roughness of the profile curve of the surface of the coating film of the magnetic recording layer by using “Surfcom-575A” (manufactured by Tokyo Seimitsu Co., Ltd.).

(16) The strength of the coating film was expressed the Young's modulus obtained by “Autograph” (produced by Shimazu Seisakusho Co., Ltd.). The Young's modulus was expressed by the ratio of the Young's modulus of the coating film to that of a commercially available video tape “AV T-120” (produce by Victor Company of Japan, Limited). The higher the relative value, the more favorable.

(17) The magnetic properties of the magnetic particles and magnetic recording medium were measured under an external magnetic field of 795.8 kA/m (10 kOe) by “Vibration Sample Magnetometer VSM-3S-15 (manufactured by Toei Kogyo, Co., Ltd.)”.

(18) The light transmittance is expressed by the linear adsorption coefficient calculated by substituting the light transmittance measured by using “UV-Vis Recording Spectrophotometer UV-2100” (manufactured by Shimazu Seisakusho, Ltd.) for the following formula. The larger the value, the more difficult it is for the magnetic recording medium to transmit light:

Linear adsorption coefficient (μm⁻¹)={l n (l/t)}/FT wherein t represents a light transmittance (−) at λ=900 nm, and FT represents thickness (μm) of the coating film used for the measurement.

(19) The surface electrical resistivity of the coating film of the magnetic recording layer was measured by the following method. That is, the coating film to be measured was exposed to the environment maintained at a temperature of 25° C. and a relative humidity of 60%, for not less than 12 hours. Thereafter, the coating film was slit into 6 mm width, and the slit coating film was placed on two metal electrodes having a width of 6.5 mm such that a coating surface thereof was contacted with the electrodes. 1.7 N (170 gw) were respectively suspended at opposite ends of the coating film so as to bring the coating film into close contact with the electrodes. D.C. 500 V was applied between the electrodes, thereby measuring the surface electrical resistivity of the coating film.

(20) The friction coefficient of the magnetic recording medium was determined by measuring a frictional force between a surface of the magnetic tape and a metal surface (aluminum polished surface) using a tensile tester TENSILON (manufactured by Shimadzu Seisakusho Co., Ltd.), and expressed by the ratio of the measured value to the load.

(21) The durability of the magnetic medium was evaluated by the following running durability and the head contamination.

The running durability was evaluated by the actual operating time under the conditions that the load was 1.96 N (200 gw) and the relative speed of the head and the tape was 16 m/s by using “Media Durability Tester MDT-3000” (manufactured by Steinberg Associates). The longer the actual operating time, the higher the running durability.

The head contamination was evaluated by visually observing the magnetic head after running the magnetic tape under a load of 1.96 N (200 gw) for 30 minutes by setting a relative speed between the magnetic tape and the magnetic head to 16 m/s, using a MEDIA DURABILITY TESTER MDT-3000 (manufactured by Steinberg Associates Co. Ltd.). The evaluation results were classified into the following four ranks. The A rank represents the smallest head contamination.

-   -   A: Not contaminated     -   B: Slightly contaminated     -   C: Contaminated     -   D: Severely contaminated

(22) The thickness of each of the non-magnetic base film, the non-magnetic undercoat layer and the magnetic recording layer constituting the magnetic recording medium was measured in the following manner by using “Digital Electronic Micrometer R351C” (manufactured by Anritsu Corp.)

The thickness (A) of a base film was first measured. Similarly, the thickness (B) (B=the sum of the thicknesses of the base film and the non-magnetic undercoat layer) of a non-magnetic substrate obtained by forming a non-magnetic undercoat layer on the base film was measured. Furthermore, the thickness (C) (C=the sum of the thicknesses of the base film, the non-magnetic undercoat layer and the magnetic recording layer) of a magnetic recording medium obtained by forming a magnetic recording layer on the non-magnetic substrata was measured. The thickness of the non-magnetic undercoat layer is expressed by (B)−(A), and the thickness of the magnetic recording layer is expressed by (C)−(B).

Meanwhile, the thicknesses of non-magnetic base film and magnetic recording layer of a magnetic recording medium were measured by the following method.

The thickness (A) of the non-magnetic base film was first measured by a digital electron micrometer “K351C” (manufactured by Anritsu Denki Co., Ltd.). After forming a magnetic recording layer on the non-magnetic base film, a thickness (B) of the thus obtained magnetic recording medium (a total thickness of the non-magnetic base film and the magnetic recording layer) was measured by the same method as used above. Then, the thickness of the magnetic recording layer is obtained by subtracting (A) from (B).

Example 1

<Production of Black Magnetic Acicular Composite Particles>

20 kg of acicular cobalt-coated magnetite particles (cobalt content: 2.38% by weight based on the weight of the acicular cobalt-coated magnetite particles; Fe²⁺ content: 15.5% by weight based on the weight of the acicular cobalt-coated magnetite particles; average major axis diameter: 0.264 μm; average minor axis diameter: 0.0334 μm; aspect ratio: 7.9:1; geometrical standard deviation value: 1.36; BET specific surface area value: 37.8 m²/g; blackness (L* value): 22.8; volume resistivity: 6.8×10⁷ Ω·cm; myristic acid absorption: 0.78 mg/m²; coercive force value: 683 Oe (54.4 kA/m); saturation magnetization value: 81.3 emu/g (81.3 Am²/kg)), were deagglomerated in 150 liters of pure water using a stirrer, and further passed through a “TK pipeline homomixer” (tradename, manufactured by Tokushu Kika Kogyo Co., Ltd.) three times, thereby obtaining a slurry containing the acicular cobalt-coated magnetite particles.

Successively, the obtained slurry containing the acicular cobalt-coated magnetite particles was passed through a transverse-type sand grinder (tradename “MIGHTY MILL MHG-1.5L”, manufactured by Inoue Seisakusho Co., Ltd.) five times at an axis-rotating speed of 2,000 rpm, thereby obtaining a slurry in which the acicular cobalt-coated magnetite particles were dispersed.

The particles in the obtained slurry which remained on a sieve of 325 meshes (mesh size: 44 μm) was 0%. The slurry was filtered and washed with water, thereby obtaining a filter cake containing the acicular cobalt-coated magnetite particles. After the obtained filter cake containing the acicular cobalt-coated magnetite particles was dried at 120° C., 11.0 kg of the dried particles were then charged into an edge runner “MPUV-2 Model” (tradename, manufactured by Matsumoto Chuzo Tekkosho Co., Ltd.), and mixed and stirred at 30 Kg/cm (294 N/cm) and a stirring speed of 22 rpm for 15 minutes, while introducing nitrogen gas thereinto at a rate of 2 liter/minute, thereby lightly deagglomerating the particles.

220 g of methyltriethoxysilane (tradename: “TSL8123”, produced by GE TOSHIBA SILICONE CO., LTD.) was mixed and diluted with 200 ml of ethanol to obtain a methyltriethoxysilane solution. The methyltriethoxysilane solution was added to the deagglomerated acicular cobalt-coated magnetite particles under the operation of the edge runner. The acicular cobalt-coated magnetite particles were continuously mixed and stirred at a linear load of 30 Kg/cm (294 N/cm) and a stirring speed of 22 rpm for 20 minutes.

Next, 550 g of carbon black fine particles A (particle shape: granular shape; average particle size: 0.022 μm; geometrical standard deviation value: 1.68; BET specific surface area value: 134 m²/g; and blackness (L* value): 16.6; pH value: 3.4; DBP oil absorption: 89 ml/100 g) were added to the acicular cobalt-coated magnetite particles coated with methyltriethoxysilane for 10 minutes while operating the edge runner. Further, the mixed particles were continuously stirred at a linear load of 30 Kg/cm (294 N/cm) and a stirring speed of 22 rpm for 30 minutes to form the carbon black coat on the coating layer composed of methyltriethoxysilane, thereby obtaining composite particles.

In order to determine the coating amount of methyltriethoxysilane and the amount of carbon black adhered, a part of the obtained composite particles were sampled and aged at 105° C. for 60 minutes using a dryer. As a result, it was confirmed that the coating amount of methyltriethoxysilane was 0.29% by weight (calculated as Si), and the amount of carbon black adhered was 4.73% by weight (corresponding to 5 parts by weight based on 100 parts by weight of the core particles). Further, as a result of the observation of electron micrograph, it was confirmed that almost whole amount of carbon black added was adhered onto the coating layer of organosilane compounds produced from methyltriethoxysilane.

Next, 220 g of dimethyl polysiloxane was added to the composite particles while operating an edge runner, and the obtained mixture was then mixed and stirred at a linear load of 60 Kg/cm (58B N/cm) for 30 minutes, thereby obtaining composite particles on the surfaces of which dimethyl polysiloxane was uniformly adhered. The stirring speed used in the above treatment was 22 rpm.

Next, 1,650 g of the above carbon black fine particles A were added to the above obtained composite particles for 10 minutes while operating the edge runner. Then, these particles were mixed and stirred with each other at a linear load of 60 Kg/cm (588 N/cm)for 30 minutes to form the second carbon black coat onto the surface of the first carbon black coat through dimethyl polysiloxane as an adhesive, thereby obtaining black magnetic acicular composite particles. The stirring speed used in the above treatment was 22 rpm.

The obtained black magnetic acicular composite particles were heat-treated at 80° C. for 120 minutes by using a. As shown in the electron micrograph, the resultant black magnetic acicular composite particles had an average major axis diameter of 0.267 μm, an average minor axis diameter of 0.0340 μm, an aspect ratio of 7.9:1. In addition, the black magnetic acicular composite particles showed a geometrical standard deviation value of 1.36, a BET specific surface area value of 40.5 m²/g, a blackness (L* value) of 18.9 and a volume resistivity of 6.4×10⁴ Ω·cm, a myristic acid-adsorption of 0.26 mg/m² and a carbon black desorption percentage of 7.1%. As to the magnetic properties, the coercive force value of the black magnetic acicular composite particles was 676 Oe (53.8 kA/m) and the saturation magnetization value was 73.2 emu/g (73.2 kA/m).

Further, it was confirmed that the total amount of carbon black adhered and bonded was 16.58% by weight (calculated as C; corresponding to 20.0 parts by weight based on 100 parts by weight of the core particles); the thickness of carbon black adhered on the surface of each particle was 0.0017 μm; and the coating amount of dimethyl polysiloxane was 0.71% by weight (calculated as Si). Meanwhile, as a result of the observation by electron microscope, it was recognized that since substantially no liberated carbon black was observed, almost whole amount of carbon black added was adhered onto the first carbon black coat.

<Production of Magnetic Recording Medium: Formation of Magnetic Recording Layer>

12 g of the thus obtained black magnetic acicular composite particles, 1.2 g of a polishing agent (AKP-30: trade name, produced by Sumitomo Chemical Co., Ltd.), 0.06 g of carbon black (#2400B, trade name, produced by Mitsubishi Chemical Corp.), a binder resin solution (30% by weight of vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group and 70% by weight of cyclohexanone) and cyclohexanone were mixed to obtain a mixture (solid content: 78% by weight). The mixture was further kneaded by a plast-mill for 30 minutes to obtain a kneaded material.

The thus-obtained kneaded material was charged into a 140 ml-glass bottle together with 95 g of 1.5 mmφ glass beads, a binder resin solution (30% by weight of polyurethane resin having a sodium sulfonate group and 70% by weight of a solvent (methyl ethyl ketone:toluene=1:1)), cyclohexanone, methyl ethyl ketone and toluene, and the mixture was mixed and dispersed by a paint shaker for 6 hours. Then, the lubricant and hardening agent were added to the mixture, and the resultant mixture was mixed and dispersed by a paint shaker for 15 minutes.

The thus-obtained magnetic coating composition was as follows:

Black magnetic acicular 100 parts by weight composite particles Vinyl chloride-vinyl acetate 10 parts by weight copolymer resin having a sodium sulfonate group Polyurethane resin having a 10 parts by weight sodium sulfonate group Polishing agent (AKP-30) 10 parts by weight Carbon black (#2400B) 0.5 parts by weight Lubricant (myristic acid: butyl 3.0 parts by weight stearate = 1:2) Hardening agent (polyisocyanate) 5.0 parts by weight Cyclohexanone 65.8 parts by weight Methyl ethyl ketone 164.5 parts by weight Toluene 98.7 parts by weight

The viscosity of the obtained magnetic coating composition was 2,202 cP.

The magnetic coating composition obtained was applied to a polyethylene terephthalate film (thickness: 12 μm) as a non-magnetic base film to a thickness of 45 μm by an applicator, and the magnetic recording medium obtained was oriented and dried in a magnetic field, and then calendered. The magnetic recording medium was then subjected to a curing reaction at 60° C. for 24 hours, and thereafter slit into a width of 0.5 inch (1.27 cm), thereby obtaining a magnetic tape. The thickness of the respective magnetic recording layer was 3.5 μm.

The coercive force value of the magnetic tape produced by forming a magnetic recording layer on the non-magnetic base film was 728 Oe (57.9 kA/m), the squareness (Br/Bm) thereof was 0.89, the gloss thereof was 176%, the surface roughness Ra thereof was 7.6 nm, the linear absorption coefficient thereof was 2.77 μm⁻¹, and the surface electrical resistivity was 1.1×10⁷ Ω/cm². The friction coefficient thereof was 0.26. Further, as to the durability of the magnetic tape, the running durability time was not less than 28.6 minutes and a friction coefficient was 0.26.

Core Particles 1 to 5:

Various magnetic acicular particles were prepared by known methods. The same procedure as defined in Example 1 was conducted by using the thus magnetic acicular particles, thereby obtaining deagglomerated magnetic acicular particles as core particles.

Various properties of the thus obtained magnetic acicular particles are shown in Table 1.

Core Particles 6:

The same procedure as defined in Example 1 was conducted by using 20 kg of the deagglomerated acicular cobalt-coated maghemite particles (core particles 1) and 150 liters of water, thereby obtaining a slurry containing the acicular cobalt-coated maghemite particles. The pH value of the obtained re-dispersed slurry containing the acicular cobalt-coated maghemite particles was adjusted to 10.5 by adding an aqueous sodium hydroxide solution, and then the concentration of the slurry was adjusted to 98 g/liter by adding water thereto. After 150 liters of the slurry was heated to 60° C., 5,444 ml of a 1.0 mol/liter sodium aluminate solution (equivalent to 1.0% by weight (calculated as Al) based on the weight of the acicular cobalt-coated maghemite particles) was added to the slurry. After allowing the slurry to stand for 30 minutes, the pH value of the slurry was adjusted to 7.5 by adding an aqueous acetic acid solution. After further allowing the slurry to stand for 30 minutes, the slurry was subjected to filtration, washing with water, drying and pulverization, thereby obtaining the acicular cobalt-coated maghemite particles coated with hydroxides of aluminum.

Main production conditions are shown in Table 2, and various properties of the obtained acicular cobalt-coated maghemite particles are shown in Table 3.

Core Particles 7 to 10:

The same procedure as defined in the production of the core particles 6 above, was conducted except that kind of core particles, and kind and amount of additives used in the surface treatment were varied, thereby obtaining surface-treated magnetic acicular particles.

Main production conditions are shown in Table 2, and various properties of the obtained surface-treated magnetic acicular particles are shown in Table 3.

In Table 2, “A” denotes a hydroxide of aluminum and “S” denotes an oxide of silicon.

Examples 2 to 13 and Comparative Examples 1 to 5

The same procedure as defined in Example 1 was conducted except that kind of core particles, addition or non-addition, kind and amount of alkoxysilane, polysiloxane or silicon compound upon the coating step, edge runner treatment conditions used in the coating step, kind and amount of carbon black fine particles added in the carbon black coat forming steps and edge runner treatment conditions used in the carbon black coat forming steps, were changed variously, thereby obtaining composite particles.

Various properties of the carbon black fine particles A to F used, are shown in Table 4. The essential treating conditions are shown in Table 5, and various properties of the obtained composite particles are shown in Table 6.

As a result of observing the composite particles obtained in Examples 2 to 13 by an electron microscope, substantially no liberated carbon black was recognized. Therefore, it was confirmed that almost whole amount of carbon black added was adhered onto the coating layer of organosilane compounds produced from alkoxysilane, or the coating layer of polysiloxane.

Meanwhile, all the additives used in Examples 4 to 6 were polysiloxanes. Specifically, “TSF484” (tradename, produced by Toshiba Silicone Co., Ltd.) was methyl hydrogen polysiloxane; “BYK-080” (tradename, produced by BYK-Chemie Japan Co., Ltd.) was modified polysiloxane; and “TSF-4770” (tradename, produced by Toshiba Silicone Co., Ltd.) was terminal carboxyl group-modified polysiloxane.

Examples 14 to 25 and Comparative Examples 6 to 12

<Production of Black Magnetic Acicular Composite Particles>

The same procedure as defined in Example 1 was conducted except that kind of composite particles, kind and amount of adhesive added in the bonding step, edge runner treatment conditions used in the bonding step, kind and amount of carbon black fine particles added in carbon black coat forming steps and edge runner treatment conditions used in the carbon black coat forming steps, were changed variously, thereby obtaining black magnetic acicular composite particles.

Meanwhile, as a result of observing the black magnetic acicular composite particles obtained in Examples 14 to 25 by an electron microscope, substantially no liberated carbon black was recognized. Therefore, it was confirmed that almost whole amount of carbon black added was adhered onto the first carbon black coat.

Main treatment conditions are shown in Table 7, and various properties of the obtained black magnetic acicular composite particles are shown in Table 8.

Examples 26 to 37 and Comparative Examples 13 to 25

<Production of Magnetic Recording Medium >

The same procedure as defined in Example 1 was conducted except for varying the kind of the black magnetic acicular composite particles, thereby producing a magnetic recording medium.

The main producing conditions and various properties are shown in Tables 9 to 10.

Example 38

<Production of Black Magnetic Acicular Composite Particles>

20 kg of acicular cobalt-coated magnetite particles (cobalt content: 2.33% by weight based on the weight of the acicular cobalt-coated magnetite particles; Fe²⁺ content: 15.4% by weight based on the weight of the acicular cobalt-coated magnetite particles; average major axis diameter: 0.270 μm; average minor axis diameter: 0.0329 μm; aspect ratio: 8.2:1; geometrical standard deviation value: 1.36; BET specific surface area value: 38.9 m²/g; blackness (L* value): 22.5; volume resistivity: 7.4×10⁷ Ω·cm; myristic acid absorption: 0.79 mg/m²; coercive force value: 698 Oe (55.5 kA/m); saturation magnetization value: 80.6 emu/g (80.6 Am²/kg)), were deagglomerated in 150 liters of pure water using a stirrer, and further passed through a “TK pipeline homomixer” (tradename, manufactured by Tokushu Kika Kogyo Co., Ltd.) three times, thereby obtaining a slurry containing the acicular cobalt-coated magnetite particles.

Successively, the obtained slurry containing the acicular cobalt-coated magnetite particles was passed through a transverse-type sand grinder (tradename “MIGHTY MILL MHG-1.5L”, manufactured by Inoue Seisakusho Co., Ltd.) five times at an axis-rotating speed of 2,000 rpm, thereby obtaining a slurry in which the acicular cobalt-coated magnetite particles were dispersed.

The particles in the obtained slurry which remained on a sieve of 325 meshes (mesh size: 44 μm) was 0%. The slurry was filtered and washed with water, thereby obtaining a filter cake containing the acicular cobalt-coated magnetite particles. After the obtained filter cake containing the acicular cobalt-coated magnetite particles was dried at 120° C., 11.0 kg of the dried particles were then charged into an edge runner “MPUV-2 Model” (tradename, manufactured by Matsumoto Chuzo Tekkosho Co., Ltd.), and mixed and stirred at 30 Kg/cm (294 N/cm) and a stirring speed of 22 rpm for 15 minutes, while introducing nitrogen gas thereinto at a rate of 2 liter/minute, thereby lightly deagglomerating the particles.

165 g of methyltriethoxysilane (tradename: “TSL8123”, produced by TOSHIBA SILICONE CO., LTD.) was mixed and diluted with 200 ml of ethanol to obtain a methyltriethoxysilane solution. The methyltriethoxysilane solution was added to the deagglomerated acicular cobalt-coated magnetite particles under the operation of the edge runner. The acicular cobalt-coated magnetite particles were continuously mixed and stirred at a linear load of 40 Kg/cm (392 N/cm) and a stirring speed of 22 rpm for 20 minutes.

Next, 825 g of carbon black fine particles B (particle shape: granular shape: average particle size: 0.022 μm; geometrical standard deviation value: 1.78; BET specific surface area value: 133.5 m²/g; and blackness (L* value): 14.6; pH value: 3.4; DBP oil absorption: 84 ml/100 g) were added to the acicular cobalt-coated magnetite particles coated with methyltriethoxysilane for 10 minutes while operating the edge runner. Further, the mixed particles were continuously stirred at a linear load of 30 Kg/cm (294 N/cm) and a stirring speed of 22 rpm for 30 minutes to form the carbon black coat on the coating layer composed of methyltriethoxysilane, thereby obtaining black magnetic acicular composite particles.

The obtained black magnetic acicular composite particles were heat-treated at 80° C. for 120 minutes by using a drier. As shown in the electron micrograph, the resultant black magnetic acicular composite particles had an average major axis diameter of 0.273 μm, an average minor axis diameter of 0.0331 μm, an aspect ratio of 8.2:1. In addition, the black magnetic acicular composite particles showed a geometrical standard deviation value of 1.36, a BET specific surface area value of 40.2 m²/g, a blackness (L* value) of 19.7 and a volume resistivity of 9.8×10⁴ Ω·cm, and a carbon black desorption percentage of 7.8%. As to the magnetic properties, the coercive force value of the black magnetic acicular composite particles was 682 Oe (53.8 kA/m) and the saturation magnetization value was 79.3 emu/g (79.3 Am²/kg).

Further, it was confirmed that the total amount of carbon black adhered and bonded was 6.88% by weight (calculated as C; corresponding to 7.5 parts by weight based on 100 parts by weight of the core particles); the thickness of carbon black coat on the surface of each particle was 0.0018 μm; and the coating amount of methyltriethoxysilane was 0.22% by weight (calculated as Si). Meanwhile, as a result of the observation by electron microscope, it was recognized that since substantially no liberated carbon black was observed, almost whole amount of carbon black added was adhered onto the coating layer of organosilane compounds produced from methyltriethoxysilane.

<Production of Non-magnetic Undercoat Layer>

12 g of non-magnetic particles 1 shown in Table 15 below (kind: hematite particles; particle shape: spindle-shaped; average major axial diameter: 0.187 μm; average minor axial diameter: 0.0240 μm; aspect ratio: 7.8:1; geometrical standard deviation value: 1.33; BET specific surface area value: 43.3 m²/g; volume resistivity value: 8.6×10⁸ Ω·cm; blackness (L* value): 32.6) were mixed with a binder resin solution (containing 30% by weight of vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group and 70% by weight of cyclohexanone) and cyclohexanone, thereby obtaining a mixture (solid content: 72%). The obtained mixture was further kneaded for 30 minutes using a plastomill, thereby obtaining a kneaded material.

The thus obtained kneaded material was added to a 140 ml glass bottle together with 95 g of 1.5 mmφ glass beads, an additional amount of a binder resin solution (containing 30% by weight of polyurethane resin having a sodium sulfonate group and 70% by weight of a mixed solvent of methyl ethyl ketone and toluene (1:1)), cyclohexanone, methyl ethyl ketone and toluene. The resultant mixture was mixed and dispersed for 6 hours by a paint shaker, thereby obtaining a coating composition. Thereafter, a lubricant was added to the obtained coating composition, and the mixture was mixed and dispersed for 15 minutes by a paint shaker.

The composition of the obtained non-magnetic coating composition was as follows:

Non-magnetic particles 1 100 parts by weight Vinyl chloride-vinyl acetate 10 parts by weight copolymer resin having a sodium sulfonate group Polyurethane resin having 10 parts by weight a sodium sulfonate group Lubricant (myristic acid: 2 parts by weight butyl stearate = 1:1) Cyclohexanone 56.9 parts by weight Methyl ethyl ketone 142.3 parts by weight Toluene 85.4 parts by weight

The obtained non-magnetic coating composition had a viscosity of 310 cP.

Next, the non-magnetic coating composition was coated on a 12 μm-thick polyethylene terephthalate film using an applicator so as to form thereon a 55 μm-thick coating layer, and then dried, thereby producing a non-magnetic undercoat layer.

The thus obtained non-magnetic undercoat layer had a thickness of 3.4 μm, and exhibited a gloss of 193%, a surface roughness Ra of 8.2 nm, a Young's modulus (relative value) of 123, a linear absorption of 1.01 μm⁻¹ and a surface electrical resistivity value of 1.1×10¹⁴ Ω·cm.

<Production of Magnetic Recording Medium: Formation of Magnetic Recording Layer>

12 g of the thus obtained black magnetic acicular composite particles, 1.2 g of a polishing agent (AKP-30: trade name, produced by Sumitomo Chemical Co., Ltd.), 0.06 g of carbon black fine particles (#2400B, trade name, produced by Mitsubishi Chemical Corp.), a binder resin solution (30% by weight of vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group and 70% by weight of cyclohexanone) and cyclohexanone were mixed to obtain a mixture (solid content: 78% by weight). The mixture was further kneaded by a plast-mill for 30 minutes to obtain a kneaded material.

The thus-obtained kneaded material was charged into a 140 ml-glass bottle together with 95 g of 1.5 mmφ glass beads, a binder resin solution (30% by weight of polyurethane resin having a sodium sulfonate group and 70% by weight of a solvent (methyl ethyl ketone:toluene=1:1)), cyclohexanone, methyl ethyl ketone and toluene, and the mixture was mixed and dispersed by a paint shaker for 6 hours. Then, the lubricant and hardening agent were added to the mixture, and the resultant mixture was mixed and dispersed by a paint shaker for 15 minutes.

The thus-obtained magnetic coating composition was as follows:

Black magnetic acicular 100 parts by weight composite particles Vinyl chloride-vinyl acetate 10 parts by weight copolymer resin having a sodium sulfonate group Polyurethane resin having a 10 parts by weight sodium sulfonate group Polishing agent (AKP-30) 10 parts by weight Carbon black fine particles 0.5 parts by weight (#2400B) Lubricant (myristic acid: butyl 3.0 parts by weight stearate = 1:2) Hardening agent (polyisocyanate) 5.0 parts by weight Cyclohexanone 65.8 parts by weight Methyl ethyl ketone 164.5 parts by weight Toluene 98.7 parts by weight

The viscosity of the obtained magnetic coating composition was 2,338 cP.

The magnetic coating composition obtained was applied to the above-mentioned non-magnetic undercoat layer to a thickness of 15 μm by an applicator, and the magnetic recording medium obtained was oriented and dried in a magnetic field, and then calendered. The magnetic recording medium was then subjected to a curing reaction at 60° C. for 24 hours, and thereafter slit into a width of 0.5 inch, thereby obtaining a magnetic tape. The thickness of the respective magnetic recording layer was 3.4 μm.

The coercive force value of the magnetic tape produced by forming a magnetic recording layer on the non-magnetic base film was 746 Oe (59.4 kA/m), the squareness (Br/Bm) thereof was 0.89, the gloss thereof was 174%, the surface roughness Ra thereof was 7.4 nm, the Young's modulus (relative value) thereof was 138, the linear absorption coefficient thereof was 1.54 μm⁻¹, and the surface electrical resistivity was 5.3×10⁷ Ω/cm². Further, as to the durability of the magnetic tape, the running durability time was not less than 29.5 minutes.

Examples 39 to 50

The same procedure as defined in Example 38 was conducted except that kind of core particles, addition or non-addition, kind and amount of alkoxysilane, polysiloxane or silicon compound upon the coating step, edge runner treatment conditions used in the coating step, kind and amount of carbon black fine particles added in the carbon black coat forming steps and edge runner treatment conditions used in the carbon black coat forming steps, were changed variously, thereby obtaining black magnetic acicular composite particles. As a result of observing the black magnetic acicular composite particles obtained in Examples 39 to 50 by an electron microscope, substantially no liberated carbon black was recognized. Therefore, it was confirmed that almost whole amount of carbon black coat was formed onto the coating layer of organosilane compounds produced from alkoxysilane, or the coating layer of polysiloxane.

The essential treating conditions are shown in Table 11, and various properties of the obtained black magnetic acicular composite particles are shown in Table 12.

Examples 51

5.0 kg of the obtained black magnetic acicular composite particles in Example 38 were charged into an edge runner “MPUV-2 Model” (tradename, manufactured by Matsumoto Chuzo Tekkosho Co., Ltd.), and then mixed and stirred at a linear load of 30 Kg/cm (294 N/cm) for 30 minutes, thereby lightly deaggregating the particles.

Then, 100 g of dimethyl polysiloxane was added to the black magnetic acicular composite particles while operating the edge runner, and mixed and stirred together at a linear load of 30 Kg/cm (294 N/cm) for 30 minutes, thereby obtaining black magnetic acicular composite particles on the surfaces of which dimethyl polysiloxane was uniformly adhered.

Then, 500 g of the above carbon black fine particles B were added to the thus obtained black magnetic acicular composite particles for 10 minutes while operating the edge runner, and further mixed and stirred together at linear load of 45 Kg/cm (441 N/cm) for 30 minutes to form a second carbon black coat onto the surface of the first carbon black coat through dimethyl polysiloxane as an adhesive, thereby obtaining black magnetic acicular composite particles.

The thus obtained black magnetic acicular composite particles were dried at 105° C. for 60 minutes using a dryer. As a result of the observation by electron microscope, it was confirmed that the thus obtained black magnetic acicular composite particles had an average major axis diameter of 0.278 μm, an average minor axis diameter of 0.0341 μm, an aspect ratio of 8.2:1. In addition, the black magnetic acicular composite particles showed a geometrical standard deviation value of 1.37, a BET specific surface area value of 43.5 m²/g, a blackness (L* value) of 18.6 and a volume resistivity of 2.1×10⁴ Ω·cm, a myristic acid-adsorption of 0.27 mg/m² and a carbon black desorption percentage of 7.6%. As to the magnetic properties, the coercive force value of the black magnetic acicular composite particles was 675 Oe (53.7 kA/m) and the saturation magnetization value was 77.4 emu/g (77.4 Am²/kg).

Further, it was confirmed that the total amount of carbon black adhered and bonded was 14.62% by weight (calculated as C; corresponding to 17.5 parts by weight based on 100 parts by weight of the core particles); the coating amount of dimethyl polysiloxane was 0.71% by weight (calculated as Si); and the thickness of carbon black adhered on the surface of each particle was 0.0020 μm. Meanwhile, as a result of the observation by electron microscope, substantially no liberated carbon black was recognized. Therefore, it was confirmed that almost whole amount of the carbon black added was adhered onto the first carbon black coat.

Examples 52 to 63

The same procedure as defined in Example 51 was conducted except that kind of black magnetic acicular composite particles, kind and amount of adhesive added in the bonding step, edge runner treatment conditions used in the bonding step, kind and amount of carbon black coat forming added in carbon black coat forming steps and edge runner treatment conditions used in the carbon black adhering steps, were changed variously, thereby obtaining black magnetic acicular composite particles.

Meanwhile, as a result of observing the black magnetic acicular composite particles obtained in Examples 52 to 63 by an electron microscope, substantially no liberated carbon black was recognized. Therefore, it was confirmed that almost whole amount of carbon black added was adhered onto the first carbon black coat.

Main treatment conditions are shown in Table 13, and various properties of the obtained black magnetic acicular composite particles are shown in Table 14.

<Production of Non-magnetic Undercoat Layers>

Non-magnetic Undercoat Layers 1 to 6:

Non-magnetic undercoat layers were produced using various non-magnetic particles by the same method as defined in Example 38.

Various properties of the non-magnetic particles 1 to 6 used above are shown in Table 15.

Main production conditions and various properties of the obtained non-magnetic undercoat layers are shown in Table 16.

Examples 64 to 88 and Comparative Examples 26 to 42

<Production of Magnetic Recording Medium>

The same procedure as defined in Example 38 was conducted except that kind and amount of black magnetic acicular composite particles added, were changed variously, thereby producing magnetic recording media.

Production conditions and various properties of magnetic recording media are shown in Tables 17 to 19.

TABLE 1 Properties of Core core particles particles Kind of core particles Particle shape Core Cobalt-coated maghemite Acicular particles 1 particles (Co content: 2.65 wt. %) Core Cobalt-coated maghemite Spindle-shaped particles 2 particles (Co content: 4.23 wt. %) Core Cobalt-coated magnetite Acicular particles 3 particles (Co content: 2.22 wt. %; Fe²⁺ content: 15.9 wt. %) Core Cobalt-coated magnetite Spindle-shaped particles 4 particles (Co content: 4.79 wt. %; Fe²⁺ content: 13.7 wt. %) Core Magnetic metal particles Spindle-shaped particles 5 containing iron as a main component (Al content: 2.80 wt. %; Co content: 5.64 wt. %) Properties of core particles Average major Average minor Core axial diameter axial diameter Aspect ratio particles (μm) (μm) (−) Core 0.273 0.0334 8.2:1 particles 1 Core 0.210 0.0285 7.4:1 particles 2 Core 0.291 0.0359 8.1:1 particles 3 Core 0.149 0.0220 6.8:1 particles 4 Core 0.125 0.0175 7.1:1 particles 5 Properties of core particles Geometrical BET standard specific deviation surface Coercive force Core value area value value particles (−) (m²/g) (kA/m) (Oe) Core 1.39 36.0 54.9 690 particles 1 Core 1.36 40.4 67.1 843 particles 2 Core 1.42 31.3 56.5 710 particles 3 Core 1.45 52.9 72.5 911 particles 4 Core 1.40 53.8 153.4 1,927 particles 5 Properties of core particles Saturation magnetization Volume Core value resistivity value particles (Am²/kg) (emu/g) (Ω· cm) Core 76.4 76.4 6.5 × 10⁸ particles 1 Core 78.7 78.7 3.6 × 10⁸ particles 2 Core 82.9 82.9 9.2 × 10⁷ particles 3 Core 81.0 81.0 5.1 × 10⁷ particles 4 Core 136.1 136.1 2.1 × 10⁷ particles 5 Properties of core particles Blackness Core (L* value) Myristic acid particles (−) adsorption (mg/m²) Core 24.3 0.78 particles 1 Core 25.6 0.83 particles 2 Core 22.9 0.86 particles 3 Core 22.8 0.88 particles 4 Core 22.4 1.01 particles 5

TABLE 2 Surface-treatment step Kind of Additives Core core Calculated Amount particles particles Kind as (wt. %) Core Core Sodium Al 1.0 particles 6 particles 1 aluminate Core Core Water glass SiO₂ 0.75 particles 7 particles 2 #3 Core Core Aluminum Al 2.0 particles 8 particles 3 sulfate SiO₂ 0.5 Water glass #3 Core Core Sodium Al 3.0 particles 9 particles 4 aluminate Core Core Water glass SiO₂ 5.0 particles 10 particles 5 #3 Surface-treatment step Coating composition Core Amount particles Kind Calculated as (wt. %) Core A Al 0.98 particles 6 Core S SiO₂ 0.72 particles 7 Core A Al 1.93 particles 8 S SiO₂ 0.46 Core A Al 2.80 particles 9 Core S SiO₂ 4.74 particles 10

TABLE 3 Properties of surface-treated magnetic acicular particles Average major Average minor Core axial diameter axial diameter Aspect ratio particles (μm) (μm) (−) Core 0.273 0.0334 8.2:1 particles 6 Core 0.210 0.0285 7.4:1 particles 7 Core 0.292 0.0360 8.1:1 particles 8 Core 0.150 0.0221 6.8:1 particles 9 Core 0.126 0.0176 7.2:1 particles 10 Properties of surface-treated magnetic acicular particles Geometrical BET standard specific deviation surface Coercive force Core value area value value particles (−) (m²/g) (kA/m) (Oe) Core 1.40 36.4 54.6 686 particles 6 Core 1.35 40.9 66.4 834 particles 7 Core 1.41 32.6 55.8 701 particles 8 Core 1.45 54.8 71.6 900 particles 9 Core 1.39 56.0 151.1 1,899 particles 10 Properties of surface-treated magnetic acicular particles Volume Saturation magnetization resistivity Core value value particles (Am²/kg) (emu/g) (Ω · cm) Core 75.9 75.9 7.2 × 10⁸ particles 6 Core 78.3 78.3 4.5 × 10⁸ particles 7 Core 81.5 81.5 1.8 × 10⁸ particles 8 Core 79.8 79.8 8.6 × 10⁷ particles 9 Core 133.2 133.2 4.3 × 10⁷ particles 10 Properties of surface-treated magnetic acicular particles Blackness Myristic acid Core (L* value) adsorption particles (−) (mg/m²) Core 24.3 0.57 particles 6 Core 25.6 0.63 particles 7 Core 23.3 0.56 particles 8 Core 23.0 0.59 particles 9 Core 22.6 0.76 particles 10

TABLE 4 Properties of carbon black fine particles Geometrical Kind of standard carbon Average deviation black fine Particle particle size value particles shape (μm) (−) Carbon Granular 0.022 1.68 black A Carbon Granular 0.022 1.78 black B Carbon Granular 0.015 1.56 black C Carbon Granular 0.030 2.06 black D Carbon Granular 0.024 1.69 black E Carbon Granular 0.028 1.71 black F Properties of carbon black fine particles Kind of BET specific carbon surface area pH DBP oil Blackness black fine value value absorption (L* value) particles (m²/g) (—) (ml/100 g) (—) Carbon 134.0  3.4  89 16.6 black A Carbon 133.5  3.4  84 14.6 black B Carbon 265.3  3.7  57 15.2 black C Carbon  84.6  8.0  95 17.0 black D Carbon 113.6 10.8 102 16.2 black E Carbon 800.0  7.0 200 15.3 black F

TABLE 5 Production of composite particles Coating step with alkoxysilane, polysiloxane or silicon compound Examples Additives and Amount added Comparative Kind of core (part by Examples particles Kind weight) Example 2 Core Methyl 2.0 particles 1 triethoxysilane Example 3 Core Methyl 1.0 particles 2 trimethoxysilane Example 4 Core TSF484 1.0 particles 3 Example 5 Core BYK-080 1.0 particles 4 Example 6 Core TSF4770 1.0 particles 5 Example 7 Core Methyl 2.0 particles 6 triethoxysilane Example 8 Core Methyl 1.5 particles 7 trimethoxysilane Example 9 Core Dimethyl 3.0 particles 8 dimethoxysilane Example 10 Core Phenyl 5.0 particles 9 triethoxysilane Example 11 Core Isobutyl 1.0 particles 10 trimethoxysilane Example 12 Core Methyl 1.0 particles 1 triethoxysilane Example 13 Core Methyl 1.0 particles 1 triethoxysilane Comparative Core — — Example 1 particles 1 Comparative Core Methyl 0.005 Example 2 particles 1 triethoxysilane Comparative Core Methyl 1.0 Example 3 particles 1 triethoxysilane Comparative Core γ-aminopropyl 1.0 Example 4 particles 1 triethoxysilane Comparative Core — — Example 5 particles 1 Production of composite particles Coating step with alkoxysilane, polysiloxane or silicon compound Coating amount Examples and Edge runner treatment (calculated as Comparative Linear load Time Si) Examples (N/cm) (Kg/cm) (min.) (wt. %) Example 2 588 60 30 0.28 Example 3 294 30 30 0.19 Example 4 441 45 20 0.42 Example 5 588 60 30 0.17 Example 6 441 45 20 0.35 Example 7 294 30 30 0.29 Example 8 588 60 30 0.29 Example 9 588 60 20 0.65 Example 10 441 45 30 0.66 Example 11 588 60 20 0.14 Example 12 294 30 20 0.15 Example 13 294 30 20 0.15 Comparative — — — — Example 1 Comparative 294 30 20 7 × 10⁻⁴ Example 2 Comparative 294 30 20 0.15 Example 3 Comparative 294 30 20 0.13 Example 4 Comparative — — — — Example 5 Production of composite particles Coat formation step with carbon black Examples and Carbon black Comparative Amount added Examples Kind (part by weight) Example 2 B 3.0 Example 3 B 10.0 Example 4 C 5.0 Example 5 C 5.0 Example 6 D 10.0 Example 7 D 5.0 Example 8 B 7.5 Example 9 C 5.0 Example 10 D 3.0 Example 11 B 10.0 Example 12 E 7.5 Example 13 F 7.5 Comparative B 5.0 Example 1 Comparative C 5.0 Example 2 Comparative D 15.0 Example 3 Comparative D 5.0 Example 4 Comparative D 10.0 Example 5 Production of composite particles. Coat formation step with carbon black Amount adhered Examples and Edge runner treatment (calculated as Comparative Linear load Time C) Examples (N/cm) (Kg/cm) (min.) (wt. %) Example 2 588 60 20 2.85 Example 3 294 30 30 9.06 Example 4 441 45 30 4.75 Example 5 588 60 30 4.72 Example 6 294 30 20 9.08 Example 7 441 45 30 4.74 Example 8 588 60 30 6.94 Example 9 294 30 20 4.75 Example 10 441 45 30 2.86 Example 11 588 60 30 9.00 Example 12 588 60 30 6.96 Example 13 588 60 30 6.83 Comparative 294 30 20 4.74 Example 1 Comparative 294 30 20 4.73 Example 2 Comparative 294 30 20 13.01 Example 3 Comparative 294 30 20 4.68 Example 4 Comparative 294 30 20 9.08 Example 5

TABLE 6 Properties of composite particles Examples and Average major Average minor Aspect Comparative axial diameter axial diameter ratio Examples (μm) (μm) (—) Example 2 0.274 0.0336 8.2:1 Example 3 0.212 0.0289 7.3:1 Example 4 0.291 0.0360 8.1:1 Example 5 0.150 0.0222 6.8:1 Example 6 0.127 0.0179 7.1:1 Example 7 0.274 0.0336 8.2:1 Example 8 0.212 0.0288 7.4:1 Example 9 0.293 0.0362 8.1:1 Example 10 0.150 0.0221 6.8:1 Example 11 0.128 0.0180 7.1:1 Example 12 0.274 0.0337 8.1:1 Example 13 0.274 0.0337 8.1:1 Comparative 0.273 0.0334 8.2:1 Example 1 Comparative 0.273 0.0334 8.2:1 Example 2 Comparative 0.275 0.0338 8.1:1 Example 3 Comparative 0.273 0.0334 8.2:1 Example 4 Comparative 0.273 0.0334 8.2:1 Example 5 Properties of composite particles Geometrical BET standard specific Examples and deviation surface Coercive force Comparative value area value value Examples (—) (m²/g) (kA/m) (Oe) Example 2 1.40 38.0 54.1 680 Example 3 1.36 43.6 66.5 836 Example 4 1.43 32.8 55.7 700 Example 5 1.44 55.3 71.7 901 Example 6 1.39 56.4 151.8 1,908 Example 7 1.40 38.6 54.0 679 Example 8 1.36 43.1 65.9 828 Example 9 1.43 33.9 55.4 696 Example 10 1.44 55.2 71.0 892 Example 11 1.39 59.0 150.3 1,889 Example 12 1.40 38.6 54.0 679 Example 13 1.40 38.7 53.8 676 Comparative — 41.8 54.0 678 Example 1 Comparative — 40.2 54.0 679 Example 2 Comparative — 42.9 53.5 672 Example 3 Comparative 41.1 54.0 678 Example 4 Comparative — 45.6 53.6 674 Example 5 Properties of composite particles Examples Volume and Saturation resistivity Blackness Comparative magnetization value value (L* value) Examples (Am²/kg) (emu/g) (Ω═cm) (—) Example 2 72.8 72.8 4.8 × 10⁴ 19.6 Example 3 73.6 73.6 9.6 × 10⁴ 19.1 Example 4 79.8 79.8 8.3 × 10⁵ 20.1 Example 5 78.1 78.1 4.6 × 10⁵ 19.8 Example 6 131.6 131.6 2.3 × 10⁵ 19.1 Example 7 72.5 72.5 2.6 × 10⁴ 19.7 Example 8 73.4 73.4 7.3 × 10⁴ 19.6 Example 9 78.8 78.8 5.1 × 10⁵ 19.4 Example 10 77.3 77.3 1.1 × 10⁵ 19.6 Example 11 130.0 130.0 2.4 × 10⁵ 18.5 Example 12 72.6 72.6 3.1 × 10⁴ 19.8 Example 13 72.5 72.5 2.2 × 10⁴ 20.1 Comparative 72.5 72.5 6.3 × 10⁷ 22.6 Example 1 Comparative 72.6 72.6 5.1 × 10⁷ 22.8 Example 2 Comparative 72.3 72.3 9.6 × 10⁶ 19.4 Example 3 Comparative 72.8 72.8 5.4 × 10⁷ 22.5 Example 4 Comparative 72.4 72.4 1.6 × 10⁷ 22.0 Examples 5 Examples Properties of composite particles and Myristic acid Carbon black Comparative adsorption desorption Examples (mg/m²) percentage (%) Example 2 0.44 8.4 Example 3 0.41 8.8 Example 4 0.46 7.2 Example 5 0.48 7.9 Example 6 0.50 8.9 Example 7 0.39 3.9 Example 8 0.37 4.2 Example 9 0.38 3.6 Example 10 0.39 2.8 Example 11 0.40 4.4 Example 12 0.51 9.1 Example 13 0.49 9.6 Comparative 0.66 65.2 Example 1 Comparative 0.67 46.6 Example 2 Comparative 0.54 28.3 Example 3 Comparative 0.71 52.8 Example 4 Comparative 0.60 68.3 Example 5

TABLE 7 Production of black magnetic acicular composite particles Treating step with dimethyl polysiloxane or silicon compound Additives Examples Amount and added Comparative Kind of core (part by Examples particles Kind weight) Example 14 Example 2 Dimethylpolysiloxane 1.0 Example 15 Example 3 Dimethylpolysiloxane 1.5 Example 16 Example 4 Dimethylpolysiloxane 2.0 Example 17 Example 5 Dimethylpolysiloxane 1.5 Example 18 Example 6 Dimethylpolysiloxane 2.0 Example 19 Example 7 Dimethylpolysiloxane 1.5 Example 20 Example 8 Dimethylpolysiloxane 1.5 Example 21 Example 9 Dimethylpolysiloxane 2.0 Example 22 Example 10 Dimethylpolysiloxane 3.0 Example 23 Example 11 Dimethylpolysiloxane 1.0 Example 24 Example 12 Dimethylpolysiloxane 2.0 Example 25 Example 13 Dimethylpolysiloxane 3.0 Comparative Comparative Dimethylpolysiloxane 1.0 Example 6 Example 1 Comparative Comparative Dimethylpolysiloxane 1.0 Example 7 Example 2 Comparative Comparative Dimethylpolysiloxane 1.0 Example 8 Example 3 Comparative Comparative Dimethylpolysiloxane 1.0 Example 9 Example 4 Comparative Example 2 — — Example 10 Comparative Example 2 Dimethylpolysiloxane 0.005 Example 11 Comparative Example 2 Methyltriethoxysilane 1.0 Example 12 Production of black magnetic acicular composite particles Treating step with dimethyl polysiloxane or silicon compound Coating amount Examples and Edge runner treatment (calculated as Comparative Linear load Time Si) Examples (N/cm) (Kg/cm) (min.) (wt. %) Example 14 588 60 30 0.31 Example 15 588 60 30 0.54 Example 16 294 30 20 0.70 Example 17 588 60 30 0.53 Example 18 588 60 20 0.72 Example 19 441 45 30 0.51 Example 20 735 75 45 0.51 Example 21 588 60 30 0.71 Example 22 588 60 25 1.01 Example 23 294 30 30 0.30 Example 24 294 30 20 0.72 Example 25 588 60 30 0.99 Comparative 588 63 30 0.31 Example 6 Comparative 588 60 30 0.30 Example 7 Comparative 588 60 30 0.30 Example 8 Comparative 588 60 30 0.31 Example 9 Comparative — — — — Example 10 Comparative 294 30 20 2 × 10⁻³ Example 11 Comparative 294 30 20 0.15 Example 12 Production of black magnetic acicular composite particles Coat formation step with carbon black Examples and Carbon black Comparative Amount added Examples Kind (part by weight) Example 14 B 10.0 Example 15 C 15.0 Example 16 D 20.0 Example 17 B 10.0 Example 18 C 10.0 Example 19 D 15.0 Example 20 B 10.0 Example 21 C 10.0 Example 22 D 20.0 Example 23 B 10.0 Example 24 E 10.0 Example 25 F 10.0 Comparative B 10.0 Example 6 Comparative C 10.0 Example 7 Comparative D 10.0 Example 8 Comparative D 10.0 Example 9 Comparative B 10.0 Example 10 Comparative B 10.0 Example 11 Comparative B 10.0 Example 12 Production of black magnetic acicular composite particles Coat formation step with carbon black Amount adhered Examples and Edge runner treatment (calculated Comparative Linear load Time as C) Examples (N/cm) (Kg/cm) (min.) (wt. %) Example 14 588 60 30 9.06 Example 15 588 60 30 13.03 Example 16 441 45 30 16.45 Example 17 441 45 20 9.01 Example 18 588 60 30 9.05 Example 19 588 60 30 13.00 Example 20 294 30 20 9.00 Example 21 588 60 30 9.06 Example 22 588 60 30 16.48 Example 23 441 45 20 9.00 Example 24 294 30 30 9.01 Example 25 588 60 30 9.03 Comparative 441 45 30 9.08 Example 6 Comparative 588 60 30 9.00 Example 7 Comparative 441 45 30 9.03 Example 8 Comparative 588 60 30 9.02 Example 9 Comparative 588 60 30 9.01 Example 10 Comparative 588 60 30 9.00 Example 11 Comparative 588 60 30 9.05 Example 12

TABLE 8 Properties of black magnetic acicular composite particles Examples and Average major Average minor Aspect Comparative axial diameter axial diameter ratio Examples (μ/m) (μm) (—) Example 14 0.276 0.0340 8.1:1 Example 15 0.214 0.0295 7.3:1 Example 16 0.294 0.0368 8.0:1 Example 17 0.152 0.0226 6.7:1 Example 18 0.129 0.0174 7.4:1 Example 19 0.277 0.0341 8.1:1 Example 20 0.214 0.0292 7.3:1 Example 21 0.295 0.0367 8.0:1 Example 22 0.154 0.0229 6.7:1 Example 23 0.130 0.0183 7.1:1 Example 24 0.276 0.0341 8.1:1 Example 25 0.276 0.0340 8.1:1 Comparative 0.274 0.0336 8.2:1 Example 6 Comparative 0.274 0.0336 8.2:1 Example 7 Comparative 0.276 0.0340 8.1:1 Example 8 Comparative 0.274 0.0336 8.2:1 Example 9 Comparative 0.274 0.0336 8.2:1 Example 10 Comparative 0.274 0.0336 8.2:1 Example 11 Comparative 0.275 0.0338 8.1:1 Example 12 Properties of biack magnetic acicular composite particles Geometrical standard BET specific Examples and deviation surface area Coercive force Comparative value value value Examples (—) (m²/g) (kA/m) (Oe) Example 14 1.40 39.8 53.5 672 Example 15 1.36 46.5 65.7 826 Example 16 1.43 36.3 54.9 690 Example 17 1.44 57.6 71.1 893 Example 18 1.39 58.9 150.9 1,896 Example 19 1.40 41.3 53.3 670 Example 20 1.36 45.3 65.3 821 Example 21 1.43 3.5.9 54.8 689 Example 22 1.44 59.1 70.3 883 Example 23 1.39 60.3 149.6 1,880 Example 24 1.40 40.1 53.4 671 Example 25 1.40 39.9 53.2 668 Comparative — 45.6 53.2 668 Example 6 Comparative — 44.8 53.2 669 Example 7 Comparative — 46.3 53.0 666 Example 8 Comparative — 45.2 53.2 668 Example 9 Comparative — 42.3 53.3 670 Example 10 Comparative — 42.2 53.3 670 Example 11 Comparative — 41.6 53.4 671 Example 12 Properties of black magnetic acicular composite particles Examples Volume and Saturation resistivity Blackness Comparative magnetization value value (L* value) Examples (Am²/kg) (emu/g) (Ω•cm) (—) Example 14 70.3 70.3 5.9 × 10³ 18.6 Example 15 71.6 71.6 9.8 × 10³ 18.1 Example 16 78.0 78.0 7.3 × 10⁴ 18.9 Example 17 76.8 76.8 6.5 × 10⁴ 18.8 Example 18 126.5 126.5 4.1 × 10⁴ 18.3 Example 19 69.8 69.8 4.8 × 10³ 19.0 Example 20 70.1 70.1 8.0 × 10³ 18.6 Example 21 76.6 76.6 6.5 × 10⁴ 18.6 Example 22 76.0 76.0 2.1 × 10⁴ 19.1 Example 23 125.6 125.6 4.1 × 10⁴ 17.9 Example 24 70.3 70.3 3.1 × 10³ 19.2 Example 25 71.6 71.6 1.3 × 10³ 19.4 Comparative 70.6 70.6 6.8 × 10⁶ 21.8 Example 6 Comparative 70.3 70.3 6.6 × 10⁶ 22.0 Example 7 Comparative 70.1 70.1 2.2 × 10⁶ 19.0 Example 8 Comparative 70.3 70.3 7.6 × 10⁶ 22.0 Example 9 Comparative 69.8 69.8 3.3 × 10⁴ 19.3 Example 10 Comparative 69.9 69.9 3.1 × 10⁴ 19.2 Example 11 Comparative 70.1 70.1 1.2 × 10⁴ 19.0 Example 12 Properties of black magnetic acicular Examples composite particles and Myristic acid Carbon black Comparative adsorption desorption Examples (mg/m²) percentage (%) Example 14 0.24 7.1 Example 15 0.22 7.5 Example 16 0.27 7.7 Example 17 0.25 6.8 Example 18 0.28 6.9 Example 19 0.21 4.1 Example 20 0.23 3.6 Example 21 0.22 3.5 Example 22 0.23 4.4 Example 23 0.24 3.1 Example 24 0.29 8.8 Example 25 0.28 9.2 Comparative 0.33 30.4 Example 6 Comparative 0.34 24.2 Exaxnple 7 Comparative 0.38 21.8 Example 8 Comparative 0.44 26.3 Example 9 Comparative 0.46 51.8 Example 10 Comparative 0.42 37.5 Example 11 Comparative 0.39 21.0 Example 12

TABLE 9 Production of magnetic Properties recording medium of coating Kind of black Weight ratio composition magnetic composite of particles Viscosity Examples acicular particles to resin (—) (cP) Example 26 Example 14 5.0:1 2,560 Example 27 Example 15 5.0:1 2,283 Example 28 Example 16 5.0:1 2,682 Example 29 Example 17 5.0:1 2,713 Example 30 Example 18 5.0:1 3,830 Example 31 Example 19 5.0:1 1,968 Example 32 Example 20 5.0:1 1,832 Example 33 Example 21 5.0:1 1,963 Example 34 Example 22 5.0:1 1,932 Example 35 Example 23 5.0:1 3,603 Example 36 Example 24 5.0:1 2,202 Example 37 Example 25 5.0:1 2,560 Properties of magnetic recording medium Thickness of magnetic Coercive force Squareness layer value (Br/Bm) Examples (μm) (kA/m) (Oe) (—) Example 26 3.5 57.4 721 0.87 Example 27 3.6 67.3 846 0.88 Example 28 3.5 55.9 703 0.88 Example 29 3.2 72.5 911 0.87 Example 30 3.8 156.2 1,963 0.87 Example 31 3.5 57.8 726 0.89 Example 32 3.3 67.5 848 0.89 Example 33 3.4 56.2. 706 0.89 Example 34 3.5 72.4 910 0.90 Example 35 3.3 155.4 1,953 0.89 Example 36 3.6 57.0 716 0.88 Example 37 3.6 57.4 721 0.88 Properties of magnetic recording medium Surface Young's modulus Gloss roughness Ra (relative Examples (%) (nm) value) Example 26 171 8.6 138 Example 27 178 7.3 134 Example 28 181 7.0 136 Example 29 176 8.1 132 Example 30 196 6.9 131 Example 31 176 7.2 141 Example 32 183 7.3 137 Example 33 188 7.0 141 Example 34 181 7.8 136 Example 35 206 8.3 136 Example 36 171 8.3 135 Example 37 173 8.3 134 Properties of magnetic recording medium Linear absorption Friction coefficient Examples (μm⁻¹) (—) Example 26 2.63 0.28 Example 27 3.11 0.26 Example 28 2.63 0.28 Example 29 2.80 0.27 Example 30 2.86 0.28 Example 31 2.63 0.22 Example 32 2.71 0.23 Example 33 2.78 0.23 Example 34 2.78 0.24 Example 35 3.18 0.24 Example 36 2.59 0.30 Example 37 2.61 0.29 Properties of magnetic recording medium Surface electrical Running durability resistivity value Examples (min.) (Ω/cm²) Example 26 26.8 2.9 × 10⁶ Example 27 27.6 6.1 × 10⁶ Example 28 26.3 9.1 × 10⁶ Example 29 26.9 1.6 × 10⁷ Example 30 26.8 8.8 × 10⁶ Example 31 ≧30 1.7 × 10⁶ Example 32 ≧30 5.6 × 10⁶ Example 33 ≧30 8.1 × 10⁶ Example 34 28.4 1.2 × 10⁷ Example 35 29.2 2.2 × 10⁷ Example 36 25.8 1.3 × 10⁶ Example 37 26.6 1.0 × 10⁶

TABLE 10 Production of magnetic recording medium Properties of Kind of Weight ratio of coating Comparative magnetic particles to composition Examples particles resin (—) Viscosity (cP) Comparative Core 5.0:1 2,813 Example 13 particles 1 Comparative Core 5.0:1 2,762 Example 14 particles 2 Comparative Core 5.0:1 2,733 Example 15 particles 3 Comparative Core 5.0:1 2,861 Example 16 particles 4 Comparative Core 5.0:1 5,206 Example 17 particles 5 Comparative Comparative 5.0:1 5,260 Example 18 Example 5 Comparative Comparative 5.0:1 4,603 Example 19 Example 6 Comparative Comparative 5.0:1 3,812 Example 20 Example 7 Comparative Comparative 5.0:1 4,163 Example 21 Example 8 Comparative Comparative 5.0:1 5,163 Example 22 Example 9 Comparative Comparative 5.0:1 3,362 Example 23 Example 10 Comparative Comparative 5.0:1 3,233 Example 24 Example 11 Comparative Comparative 5.0:1 3,168 Example 25 Example 12 Properties of magnetic recording medium Thickness of magnetic Coercive force Squareness Comparative layer value (Br/Bm) Examples (μm) (kA/m) (Oe) (—) Comparative 3.6 57.8 726 0.85 Example 13 Comparative 3.5 66.9 841 0.85 Example 14 Comparative 3.S 56.2 706 0.84 Example 15 Comparative 3.5 72.9 916 0.85 Example 16 Comparative 3.8 158.8 1,996 0.87 Example 17 Comparative 3.6 56.8 714 0.79 Example 18 Comparative 3.5 56.7 713 0.83 Example 19 Comparative 3.6 57.1 718 0.82 Example 20 Comparative 3.7 56.7 712 0.81 Example 21 Comparative 3.7 57.0 716 0.81 Example 22 Comparative 3.5 56.4 709 0.83 Example 23 Comparative 3.5 56.6 711 0.82 Example 24 Comparative 3.5 56.7 713 0.82 Example 25 Properties of magnetic recording medium Surface Young's modulus Comparative Gloss roughness Ra (relative Examples (%) (nm) value) Comparative 165 10.4 134 Example 13 Comparative 166 10.4 133 Example 14 Comparative 162 10.8 134 Example 15 Comparative 167 11.2 132 Example 16 Comparative 195 13.6 129 Example 17 Comparative 138 21.2 123 Example 18 Comparative 155 15.3 126 Example 19 Comparative 158 14.6 126 Example 20 Comparative 159 13.8 125 Example 21 Comparative 156 14.0 126 Example 22 Comparative 143 18.3 124 Example 23 Comparative 146 16.6 124 Example 24 Comparative 151 15.3 126 Example 25 Properties of magnetic recording medium Comparative Linear absorption Friction coefficient Examples (μm⁻¹) (—) Comparative 0.72 0.36 Example 13 Comparative 0.68 0.38 Example 14 Comparative 1.10 0.38 Example 15 Comparative 1.02 0.36 Example 16 Comparative 1.12 0.41 Example 17 Comparative 1.68 0.41 Example 18 Comparative 1.63 0.36 Example 19 Comparative 1.61 0.37 Example 20 Comparative 1.72 0.38 Example 21 Comparative 1.70 0.42 Example 22 Comparative 1.61 0.43 Example 23 Comparative 1.62 0.41 Example 24 Comparative 1.68 0.39 Example 25 Properties of magnetic recording medium Surface electrical Comparative Running durability resistivity value Examples (min.) (Ω/cm²) Comparative 18.5 6.9 × 10¹² Example 13 Comparative 17.6 1.8 × 10¹³ Example 14 Comparative 17.2 9.2 × 10¹¹ Example 15 Comparative 18.3 4.1 × 10¹¹ Example 16 Comparative 16.4 3.6 × 10¹¹ Example 17 Comparative 8.3 1.6 × 10⁸ Example 18 Comparative 10.6 8.3 × 10⁸ Example 19 Comparative 9.8 7.2 × 10⁸ Example 20 Comparative 11.1 1.4 × 10⁸ Example 21 Comparative 9.9 6.9 × 10⁸ Example 22 Comparative 13.6 4.3 × 10⁸ Example 23 Comparative 14.3 3.6 × 10⁸ Example 24 Comparative 14.6 3.1 × 10⁸ Example 25

TABLE 11 Production of black magnetic acicular composite particles Coating step with alkoxysilane, polysiloxane or silicon compound Additives Amount added Kind of core (part by Examples particles Kind weight) Example 39 Core Methyl 1.0 particles 1 triethoxysilane Example 40 Core Methyl 1.5 particles 2 trimethoxysilane Example 41 Core Dimethyl 2.5 particles 3 dimethoxysilane Example 42 Core Phenyl 2.0 particles 4 triethoxysilane Example 43 Core Isobutyl 3.0 particles 5 tirmethoxysilane Example 44 Core Methyl 1.5 particles 6 triethoxysilane Example 45 Core Methyl 2.0 particles 7 trimethoxysilane Example 46 Core TSF484 1.0 particles 8 Example 47 Core BYK-080 1.0 particles 9 Example 48 Core TSF4770 1.0 particles 10 Example 49 Core Methyl 2.5 particles 1 triethoxysilane Example 50 Core Methyl 2.0 particles 1 triethoxysilane Production of black magnetic acicular composite particles Coating step with alkoxysilane, polysiloxane or silicon compound Coating amount Edge runner treatment (calculated Linear load Time as Si) Examples (N/cm) (Kg/cm) (min.) (wt. %) Example 39 294 30 30 0.14 Example 40 441 45 30 0.29 Example 41 588 60 30 0.55 Example 42 441 45 20 0.25 Example 43 392 40 25 0.44 Example 44 441 45 30 0.22 Example 45 588 60 20 0.39 Example 46 294 30 20 0.42 Example 47 294 30 30 0.17 Example 48 441 45 30 0.35 Example 49 588 60 20 0.37 Example 50 294 30 30 0.39 Production of black magnetic acicular composite particles Coat formation step with carbon black Carbon black Amount added Examples Kind (part by weight) Example 39 B 5.0 Example 40 B 7.5 Example 41 C 3.0 Example 42 C 10.0 Example 43 D 8.0 Example 44 D 10.0 Example 45 B 5.0 Example 46 C 7.5 Example 47 D 5.0 Example 48 B 3.0 Example 49 E 5.0 Example 50 F 5.0 Production of black magnetic acicular composite particles Coat formation step with carbon black Amount adhered Edge runner treatment (calculated Linear load Time as C) Examples (N/cm) (Kg/cm) (min.) (wt. %) Example 39 441 45 20 4.75 Example 40 588 60 30 6.93 Example 41 294 30 30 2.85 Example 42 588 60 20 9.00 Example 43 441 45 20 7.35 Example 44 392 40 25 9.01 Example 45 294 30 30 4.74 Example 46 588 60 20 6.92 Example 47 441 45 20 4.74 Example 48 588 60 20 2.82 Example 49 588 60 20 4.75 Example 50 588 60 20 4.73

TABLE 12 Properties of black magnetic acicular composite particles Average major Average minor Aspect axial diameter axial diameter ratio Examples (μm) (μm) (—) Example 39 0.276 0.0336 8.2:1 Example 40 0.211 0.0288 7.3:1 Example 41 0.291 0.0360 8.1:1 Example 42 0.151 0.0224 6.7:1 Example 43 0.127 0.0179 7.1:1 Example 44 0.277 0.0338 8.2:1 Example 45 0.212 0.0288 7.4:1 Example 46 0.293 0.0362 8.1:1 Example 47 0.150 0.0221 6.8:1 Example 48 0.129 0.0180 7.2:1 Example 49 0.274 D.0337 8.1:1 Example 50 0.274 0.0337 8.1:1 Properties of black magnetic acicular composite particles Geometrical BET standard specific deviation surface Coercive force value area value value Examples (—) (m²/g) (kA/m) (Oe) Example 39 1.40 38.6 53.9 677 Example 40 1.35 43.0 66.8 839 Example 41 1.43 32.2 55.9 702 Example 42 1.43 56.2 70.7 889 Example 43 1.39 55.9 152.3 1,914 Example 44 1.40 40.1 53.0 666 Example 45 1.36 42.4 66.1 831 Example 46 1.43 34.7 54.9 690 Example 47 1.45 55.7 70.4 885 Example 48 1.39 57.0 151.4 1,902 Example 49 1.40 38.1 54.4 683 Example 50 1.40 39.3 54.2 681 Properties of black magnetic acicular composite particles Saturation magnetization Volume value resistivity Examples (Am²/kg) (emu/g) value (Ω•cm) Example 39 72.4 72.4 4.1 × 10⁴ Example 40 74.0 74.0 1.5 × 10⁵ Example 41 80.1 80.1 7.5 × 10⁵ Example 42 76.8 76.8 1.6 × 10⁵ Example 43 132.0 132.0 3.9 × 10⁵ Example 44 70.7 70.7 1.7 × 10⁴ Example 45 74.1 74.1 8.6 × 10⁴ Example 46 78.2 78.2 4.4 × 10⁵ Example 47 76.9 76.9 9.3 × 10⁴ Example 48 131.8 131.8 5.2 × 10⁵ Example 49 72.9 72.9 4.1 × 10⁴ Example 50 72.7 72.7 2.6 × 10⁴ Properties of black magnetic acicular composite particles Blackness Carbon black (L* value) desorption Examples (—) percentage (%) Example 39 19.4 8.5 Example 40 19.3 8.5 Example 41 20.2 7.1 Example 42 19.5 8.4 Example 43 19.2 8.9 Example 44 19.4 4.3 Example 45 19.8 4.0 Example 46 19.2 3.8 Example 47 19.5 2.9 Example 48 18.9 3.6 Example 49 20.0 9.0 Example 50 20.3 9.5

TABLE 13 Production of black magnetic acicular composite particles Treating step with dimethyl polysiloxane Additives Amount added Kind of core (part by Examples particles Kind weight) Example 51 Example 38 Dimethyl 2.0 polysiloxane Example 52 Example 39 Dimethyl 1.5 polysiloxane Example 53 Example 40 Dimethyl 1.5 polysiloxane Example 54 Example 41 Dimethyl 2.0 polysiloxane Example 55 Example 42 Dimethyl 3.0 polysiloxane Example 56 Example 43 Dimethyl 1.0 polysiloxane Example 57 Example 44 Dimethyl 1.0 polysiloxane Example 58 Example 45 Dimethyl 1.5 polysiloxane Example 59 Example 46 Dimethyl 2.0 polysiloxane Example 60 Example 47 Dimethyl 1.5 polysiloxane Example 61 Example 48 Dimethyl 2.0 polysiloxane Example 62 Example 49 Dimethyl 1.5 polysiloxane Example 63 Example 50 Dimethyl 1.0 polysiloxane Production of black magnetic acicular composite particles Treating step with dimethyl polysiloxane Coating amount Edge runner treatment (calculated Linear load Time as Si) Examples (N/cm) (Kg/cm) (min.) (wt. %) Example 51 294 30 30 0.71 Example 52 441 45 20 0.51 Example 53 735 75 20 0.50 Example 54 588 60 20 0.70 Example 55 392 40 25 1.01 Example 56 294 30 30 0.30 Example 57 441 45 30 0.31 Example 58 588 60 20 0.52 Example 59 588 60 20 0.70 Example 60 588 60 20 0.53 Example 61 441 45 30 0.72 Example 62 588 60 30 0.51 Example 63 294 30 25 0.30 Production of black magnetic acicular composite particles Coat formation step with carbon black Carbon black Amount added Examples Kind (part by weight) Example 51 B 10.0 Example 52 B 15.0 Example 53 C 5.0 Example 54 D 10.0 Example 55 B 15.0 Example 56 C 15.0 Example 57 D 10.0 Example 58 B 15.0 Example 59 C 7.5 Example 60 D 10.0 Example 61 B 20.0 Example 62 E 15.0 Example 63 F 15.0 Production of black magnetic acicular composite particles Coat formation step with carbon black Amount adhered Edge runner treatment (calculated Linear load Time as C) Examples (N/cm) (Kg/cm) (min.) (wt. %) Example 51 441 45 30 9.02 Example 52 588 60 20 13.00 Example 53 588 60 20 4.72 Example 54 294 30 25 9.01 Example 55 588 60 20 13.01 Example 56 441 45 30 12.95 Example 57 588 60 20 8.99 Example 58 294 30 30 13.01 Example 59 294 30 30 6.91 Example 60 588 60 20 9.00 Example 61 441 45 30 16.45 Example 62 294 30 40 12.86 Example 63 588 60 30 12.99

TABLE 14 Properties of black magnetic acicular composite particles Average major Average minor Aspect Examples axial diameter axial diameter ratio Example 51 0.278 0.0341 8.2:1 Example 52 0.279 0.0342 8.2:1 Example 53 0.213 0.0290 7.3:1 Example 54 0.293 0.0364 8.0:1 Example 55 0.154 0.0230 6.7:1 Example 56 0.131 0.0185 7.1:1 Example 57 0.279 0.0342 8.2:1 Example 58 0.215 0.0294 7.3:1 Example 59 0.295 0.0367 6.0:1 Example 60 0.152 0.0225 6.8:1 Example 61 0.133 0.0188 7.1:1 Example 62 0.277 0.0343 8.1:1 Example 63 0.277 0.0342 8.1:1 Properties of black magnetic acicular composite particles Geometrical BET standard specific deviation surface Coercive force value area value value Examples (—) (m²/g) (kA/m) (Oe) Example 51 1.37 43.5 53.7 675 Example 52 1.41 40.2 52.4 659 Example 53 1.35 44.3 66.1 830 Example 54 1.43 34.2 55.2 694 Example 55 1.44 59.0 69.7 876 Example 56 1.40 58.1 150.6 1,893 Example 57 1.39 41.7 51.9 652 Example 58 1.37 45.2 65.0 817 Example 59 1.42 35.6 54.1 680 Example 60 1.44 57.0 69.9 878 Example 61 1.40 59.9 149.7 1,881 Example 62 1.39 40.5 53.1 667 Example 63 1.40 43.4 52.9 665 Properties of black magnetic acicular composite particles Volume Saturation resistivity Blackness magnetization value value (L* value) Examples (Am²/kg) (emu/g) (Ω•cm) (—) Example 51 77.4 77.4 2.1 × 10⁴ 18.6 Example 52 70.1 70.1 5.1 × 10³ 18.4 Example 53 73.2 73.2 6.5 × 10⁴ 18.5 Example 54 79.4 79.4 8.3 × 10⁴ 19.1 Example 55 84.8 84.8 5.9 × 10⁴ 18.5 Example 56 130.6 130.6 2.9 × 10⁴ 18.2 Example 57 69.5 69.5 7.3 × 10³ 18.8 Example 58 72.9 72.9 9.9 × 10³ 18.9 Example 59 76.8 76.8 6.6 × 10⁴ 18.2 Example 60 75.9 75.9 8.8 × 10³ 19.1 Example 61 129.3 129.3 4.1 × 10⁴ 18.7 Example 62 71.1 71.1 3.2 × 10³ 19.1 Example 63 70.8 70.8 2.4 × 10³ 19.3 Properties of black magnetic acicular composite particles Carbon black desorption Myristic acid adsorption percentage Examples (mg/m²) (%) Example 51 0.27 7.6 Example 52 0.24 6.9 Example 53 0.24 7.2 Example 54 0.27 7.9 Example 55 0.24 7.0 Example 56 0.27 6.7 Example 57 0.21 3.9 Example 58 0.23 3.8 Example 59 0.23 3.4 Example 60 0.22 3.6 Example 61 0.24 4.4 Example 62 0.29 9.2 Example 63 0.28 9.3

TABLE 15 Properties of non-magnetic particles for Kind of non- non-magnetic undercoat layer magnetic particles Kind Particle shape Non-magnetic Hematite particles Spindle-shaped particles 1 Non-magnetic Goethite particles Acicular particles 2 Non-magnetic Hematite particles Acicular particles 3 Non-magnetic Hematite particles Acicular particles 4 Non-magnetic Hematite particles Acicular particles 5 Non-magnetic Goethite particles Acicular particles 6 Properties of non-magnetic particles for non-magnetic undercoat layer Average major Average minor Aspect Kind of non- axial diameter axial diameter ratio magnetic particles (μm) (μm) (—) Non-magnetic 0.187 0.0240 7.8:1 particles 1 Non-magnetic 0.240 0.0272 8.8:1 particles 2 Non-magnetic 0.143 0.0210 6.8:1 particles 3 Non-magnetic 0.115 0.0179 6.4:1 particles 4 Non-magnetic 0.143 0.0211 6.8:1 particles 5 Non-magnetic 0.240 0.0273 8.8:1 particles 6 Properties of non-magnetic particles for non-magnetic undercoat layer Geometrical BET specific Kind of non- standard deviation surface area value magnetic particles value (—) (m²/g) Non-magnetic 1.33 43.3 particles 1 Non-magnetic 1.37 86.3 particles 2 Non-magnetic 1.37 54.9 particles 3 Non-magnetic 1.35 58.3 particles 4 Non-magnetic 1.37 55.6 particles 5 Non-magnetic 1.35 88.1 particles 6 Properties of non-magnetic particles for non-magnetic undercoat layer Coating Amount of carbon amount of Al black adhered Kind of non- Al content (calculated as C) magnetic particles (wt. %) (wt. %) (wt. %) Non-magnetic — — — particles 1 Non-magnetic — — — particles 2 Non-magnetic 0.98 — — particles 3 Non-magnetic — 0.67 — particles 4 Non-magnetic — — 4.75 particles 5 Non-magnetic — — 4.81 particles 6 Properties of non-magnetic particles for non-magnetic undercoat layer Volume resistivity Blackness Kind of non- value (L* value) magnetic particles (Ω•cm) (—) Non-magnetic 8.6 × 10⁸ 32.6 particles 1 Non-magnetic 9.6 × 10⁷ 34.6 particles 2 Non-magnetic 4.6 × 10⁸ 28.4 particles 3 Non-magnetic 3.2 × 10⁸ 29.6 particles 4 Non-magnetic 3.6 × 10⁴ 18.5 particles 5 Non-magnetic 5.8 × 10³ 20.3 particles 6

TABLE 16 Production of non-magnetic undercoat layer Properties Weight ratio of coating of particles composition Undercoat Kind of non- to resin Viscosity layer magnetic particles (—) (cP) Undercoat Non-magnetic 5.0:1 310 layer 1 particles 1 Undercoat Non-magnetic 5.0:1 1,139 layer 2 particles 2 Undercoat Non-magnetic 5.0:1 448 layer 3 particles 3 Undercoat Non-magnetic 5.0:1 403 layer 4 particles 4 Undercoat Non-magnetic 5.0:1 399 layer 5 particles 5 Undercoat Non-magnetic 5.0:1 1,336 layer 6 particles 6 Properties of nonmagnetic undercoat layer Surface Undercoat Thickness Gloss roughness Ra layer (μm) (%) (nm) Undercoat 3.4 193 8.2 layer 1 Undercoat 3.5 180 12.0 layer 2 Undercoat 3.4 205 6.3 layer 3 Undercoat 3.4 211 6.2 layer 4 Undercoat 3.4 199 7.1 layer 5 Undercoat 3.5 185 9.0 layer 6 Properties of non-magnetic undercoat layer Surface electrical Young's modulus Linear resistivity Undercoat (relative absorption value layer value) (μm⁻¹) (Ω/cm²) Undercoat 123 1.01 1.1 × 10¹⁴ layer 1 Undercoat 125 0.79 2.1 × 10¹³ layer 2 Undercoat 126 1.01 3.5 × 10¹³ layer 3 Undercoat 125 0.98 3.6 × 10¹³ layer 4 Undercoat 125 1.52 4.1 × 10⁹ layer 5 Undercoat 129 1.46 2.3 × 10¹⁰ layer 6

TABLE 17 Production of magnetic recording medium Properties Kind of of coating Kind of non- magnetic composition magnetic undercoat acicular Viscosity Examples layer particles (cP) Example 64 Undercoat layer 1 Example 39 2,304 Example 65 Undercoat layer 2 Example 40 2,816 Example 66 Undercoat layer 3 Example 41 2,560 Example 67 Undercoat layer 4 Example 42 2,944 Example 68 Undercoat layer 5 Example 43 6,400 Example 69 Undercoat layer 6 Example 44 2,176 Example 70 Undercoat layer 1 Example 45 2,252 Example 71 Undercoat layer 2 Example 46 2,048 Example 72 Undercoat layer 3 Example 47 2,406 Example 73 Undercoat layer 4 Example 48 5,376 Example 74 Undercoat layer 5 Example 49 2,150 Example 75 Undercoat layer 6 Example 50 2,022 Properties of magnetic recording medium Thickness of magnetic Coercive force Squareness layer value (Br/Bm) Examples (μm) (kA/m) (Oe) (—) Example 64 1.0 59.0 741 0.88 Example 65 1.0 71.9 903 0.89 Example 66 1.1 61.1 768 0.88 Example 67 1.0 76.9 966 0.88 Example 68 1.2 157.7 1,982 0.88 Example 69 1.0 59.1 743 0.89 Example 70 1.1 72.1 906 0.89 Example 71 1.1 61.3 770 0.90 Example 72 1.1 77.1 969 0.90 Example 73 1.1 157.8 1,983 0.89 Example 74 1.0 59.3 745 0.88 Example 75 1.0 59.5 748 0.88 Properties of magnetic recording medium Surface Young's modulus Gloss roughness Ra (relative Examples (%) (nm) value) Example 64 196 6.3 138 Example 65 188 7.1 141 Example 66 210 6.1 143 Example 67 214 6.4 141 Example 68 234 6.0 140 Example 69 198 6.0 142 Example 70 189 6.4 141 Example 71 213 5.8 142 Example 72 217 5.7 145 Example 73 235 5.6 142 Example 74 201 6.6 143 Example 75 193 6.8 145 Properties of magnetic recording medium Surface Linear Running electrical absorption durability resistivity Examples (μm⁻¹) (min.) value (Ω/cm²) Example 64 1.54 26.9 6.6 × 10⁷ Example 65 1.51 27.3 8.3 × 10⁷ Example 66 1.51 27.1 9.3 × 10⁷ Example 67 1.55 27.3 1.8 × 10⁷ Example 68 1.65 26.4 2.3 × 10⁷ Example 69 1.71 29.8 5.4 × 10⁷ Example 70 1.56 28.9 5.8 × 10⁷ Example 71 1.58 29.3 2.3 × 10⁷ Example 72 1.61 27.8 8.8 × 10⁷ Example 73 1.66 28.8 9.3 × 10⁷ Example 74 1.67 26.1 2.3 × 10⁷ Example 75 1.66 25.8 1.6 × 10⁷

TABLE 18 Production of magnetic recording medium Properties Kind of of coating Kind of non- magnetic composition magnetic undercoat acicular Viscosity Examples layer particles (cP) Example 76 Undercoat layer 1 Example 51 2,560 Example 77 Undercoat layer 1 Example 52 2,184 Example 78 Undercoat layer 2 Example 53 2,406 Example 79 Undercoat layer 3 Example 54 2,150 Example 80 Undercoat layer 4 Example 55 2,560 Example 81 Undercoat layer 5 Example 56 3,844 Example 82 Undercoat layer 6 Example 57 2,022 Example 83 Undercoat layer 1 Example 58 2,048 Example 84 Undercoat layer 2 Example 59 2,150 Example 85 Undercoat layer 3 Example 60 1,864 Example 86 Undercoat layer 4 Example 61 3,844 Example 87 Undercoat layer 5 Example 62 1,840 Example 88 Undercoat layer 6 Example 63 1,768 Properties of magnetic recording medium Thickness of magnetic Coercive force Squareness layer value (Br/Bm) Examples (μm) (kA/m) (Oe) (-) Example 76 1.1 58.8 739 0.88 Example 77 1.0 59.1 743 0.88 Example 78 1.0 71.8 902 0.88 Example 79 1.0 60.9 765 0.89 Example 80 1.0 76.3 959 0.89 Example 81 1.1 157.6 1,980 0.88 Example 82 1.1 59.4 747 0.89 Example 83 1.0 72.7 913 0.90 Example 84 1.0 62.2 781 0.91 Example 85 1.0 76.9 966 0.89 Example 86 1.1 157.6 1,980 0.90 Example 87 1.0 59.1 743 0.89 Example 88 1.0 59.7 750 0.89 Properties of magnetic recroding medium Surface Young's modulus Gloss roughness Ra (relative Examples (%) (nm) value) Example 76 196 6.2 138 Example 77 193 6.1 139 Example 78 191 6.9 140 Example 79 213 6.2 141 Example 80 214 6.4 143 Example 81 236 5.9 141 Example 82 195 5.8 143 Example 83 193 6.1 142 Example 84 215 5.6 143 Example 85 219 5.6 146 Example 86 238 5.6 146 Example 87 203 6.5 143 Example 88 196 6.6 143 Properties of magnetic recording medium Linear absorption Friction coefficient Examples (μm⁻¹) (-) Example 76 2.89 0.28 Example 77 2.91 0.27 Example 78 2.82 0.26 Example 79 2.86 0.26 Example 80 2.88 0.28 Example 81 3.21 0.28 Example 82 3.03 0.23 Example 83 2.91 0.24 Example 84 2.80 0.22 Example 85 2.92 0.21 Example 86 2.89 0.24 Example 87 3.01 0.30 Example 88 2.99 0.29 Properties of magnetic recording medium Running Surface electrical durability resistivity value Examples (min.) (Ω/cm²) Example 76 ≧30 4.1 × 10⁶ Example 77 28.9 1.8 × 10⁶ Example 78 28.6 3.1 × 10⁶ Example 79 28.0 6.3 × 10⁶ Example 80 ≧30 1.3 × 10⁶ Example 81 27.9 6.8 × 10⁵ Example 82 ≧30 7.5 × 10⁵ Example 83 ≧30 2.3 × 10⁶ Example 84 ≧30 4.1 × 10⁶ Example 85 ≧30 9.8 × 10⁵ Example 86 ≧30 1.2 × 10⁶ Example 87 27.1 5.5 × 10⁵ Example 88 26.5 4.0 × 10⁵

TABLE 19 Production of magnetic recording medium Properties of Kind of non- coating magnetic Kind of composition Comparative undercoat magnetic Viscosity Examples layer particles (cP) Comparative Undercoat Core particles 3,218 Example 26 layer 1 1 Comparative Undercoat Core particles 2,813 Example 27 layer 1 2 Comparative Undercoat Core particles 3,384 Example 28 layer 1 3 Comparative Undercoat Core particles 2,813 Example 29 layer 1 4 Comparative Undercoat Core particles 10,240 Example 30 layer 1 5 Comparative Undercoat Comparative 3,813 Example 31 layer 1 Example 1 Comparative Undercoat Comparative 3,618 Example 32 layer 1 Example 2 Comparative Undercoat Coinparative 3,532 Example 33 layer 1 Example 3 Comparative Undercoat Comparative 3,653 Exanple 34 layer 1 Example 4 Comparative Undercoat Comparative 4,210 Example 35 layer 1 Example 5 Comparative Undercoat Comparative 3,402 Example 36 layer 1 Example 6 Comparative Undercoat Comparative 3,821 Example 37 layer 1 Example 7 Comparative Undercoat Comparative 3,836 Example 38 layer 1 Example 8 Comparative Undercoat Comparative 3,306 Example 39 layer 1 Example 9 Comparative Undercoat Comparative 3,413 Example 40 layer 1 Example 10 Comparative Undercoat Comparative 3,526 Example 41 layer 1 Example 11 Comparative Undercoat Comparative 3,600 Example 42 layer 1 Example 12 Properties of magnetic recording medium Thickness of magnetic Coercive force Squareness Comparative layer value (Br/Bm) Examples (μm) (kA/m) (Oe) (-) Comparative 1.0 58.7 738 0.84 Example 26 Comparative 1.0 70.3 883 0.83 Example 27 Comparative 1.0 59.0 742 0.83 Example 28 Comparative 1.0 75.3 946 0.80 Example 29 Comparative 1.2 156.0 1,960 0.85 Example 30 Comparative 1.0 58.2 731 0.82 Example 31 Comparative 1.0 58.6 736 0.84 Example 32 Comparative 1.0 58.7 738 0.83 Example 33 Comparative 1.1 59.0 741 0.85 Example 34 Comparative 1.0 58.6 736 0.80 Example 35 Comparative 1.0 58.3 733 0.82 Example 36 Comparative 1.0 57.9 728 0.81 Example 37 Comparative 1.1 58.6 736 0.83 Example 38 Comparative 1.0 59.4 746 0.81 Example 39 Comparative 1.1 57.5 723 0.83 Example 40 Comparative 1.0 58.6 736 0.84 Example 41 Comparative 1.0 58.5 735 0.84 Example 42 Properties of magnetic recording medium Surface Young's modulus Comparative Gloss roughness Ra (relative Examples (%) (nm) value) Comparative 164 10.2 127 Example 26 Comparative 171 10.2 128 Example 27 Comparative 168 10.6 128 Example 28 Comparative 160 11.1 127 Example 29 Comparative 196 13.0 126 Example 30 Comparative 163 18.4 128 Example 31 Comparative 168 17.3 128 Example 32 Comparative 169 16.4 128 Example 33 Comparative 163 21.3 128 Example 34 Comparative 160 21.6 126 Example 35 Comparative 164 15.5 127 Example 36 Comparative 165 19.6 127 Example 37 Comparative 168 15.2 128 Example 38 Comparative 163 15.6 128 Example 39 Comparative 160 18.9 128 Example 40 Comparative 162 20.1 128 Example 41 Comparative 169 14.6 128 Example 42 Properties of magnetic recording medium Comparative Linear absorption Friction coefficient Examples (μm⁻¹) (-) Comparative 1.06 0.44 Example 26 Comparative 1.08 0.43 Example 27 Comparative 1.11 0.43 Example 28 Comparative 1.07 0.42 Example 29 Comparative 1.19 0.51 Example 30 Comparative 1.16 0.46 Example 31 Comparative 1.18 0.43 Example 32 Comparative 1.19 0.42 Example 33 Comparative 1.20 0.45 Example 34 Comparative 1.20 0.46 Example 35 Comparative 1.75 0.41 Example 36 Comparative 1.76 0.43 Example 37 Comparative 1.78 0.46 Example 38 Comparative 1.79 0.48 Example 39 Comparative 1.79 0.43 Example 40 Comparative 1.76 0.41 Example 41 Comparative 1.76 0.46 Example 42 Properties of magnetic recording medium Running Surface electrical Comparative durability resistivity value Examples (min.) (Ω/cm²) Comparative 19.2 4.8 × 10¹² Example 26 Comparative 18.6 7.4 × 10¹² Example 27 Comparative 18.0 7.3 × 10¹¹ Example 28 Comparative 19.1 1.4 × 10¹¹ Example 29 Comparative 17.2 1.8 × 10¹¹ Example 30 Comparative 9.6 5.8 × 10¹¹ Example 31 Comparative 8.3 3.2 × 10¹¹ Example 32 Comparative 6.4 1.1 × 10¹¹ Example 33 Comparative 5.8 2.6 × 10¹¹ Example 34 Comparative 6.1 8.6 × 10¹⁰ Example 35 Comparative 10.5 7.6 × 10⁸  Example 36 Comparative 11.1 5.8 × 10⁸  Example 37 Comparative 13.1 3.1 × 10⁸  Example 38 Comparative 11.4 4.8 × 10⁸  Example 39 Comparative 15.1 5.1 × 10⁸  Example 40 Comparative 16.2 4.6 × 10⁸  Example 41 Comparative 16.6 2.9 × 10⁸  Example 42 

1. A magnetic recording medium comprising: a non-magnetic base film; and a magnetic recording layer comprising a binder resin and black magnetic acicular composite particles having an average particle diameter of 0.051 to 0.35 μm, comprising magnetic acicular cobalt-coated iron oxide particles or magnetic acicular metal particles containing iron as a main component, a coating formed on surface of said magnetic acicular particles, comprising at least one organosilicon compound selected from the group consisting of: (1) organosilane compounds obtainable from alkoxysilane compounds, and (2) polysiloxanes or modified polysiloxanes, and a carbon black coat formed on said coating layer comprising said organosilicon compound, in an amount of from more than 10 to 40 parts by weight based on 100 parts by weight of said magnetic acicular particles.
 2. A magnetic recording medium according to claim 1, wherein said magnetic acicular particles are particles having a coat which is formed on at least a part of the surface of said magnetic acicular particles and which comprises at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of 0.01 to 20% by weight, calculated as Al or SiO₂, based on the total weight of the magnetic acicular particles coated.
 3. A magnetic recording medium according to claim 1, wherein said modified polysiloxanes are compounds selected from the group consisting of: (A) polysiloxanes modified with at least one compound selected from the group consisting of polyethers, polyesters and epoxy compounds, and (B) polysiloxanes whose molecular terminal is modified with at least one group selected from the group consisting of carboxylic acid groups, alcohol groups and a hydroxyl group.
 4. A magnetic recording medium according to claim 1, wherein said alkoxysilane compound is represented by the general formula (I): R¹ _(a)SiX_(4−a)  (I) wherein R¹ is C₆H₅—, (CH₃)₂CHCH₂— or n-C_(b)H_(2b+1)— (wherein b is an integer of 1 to 18); X is CH₃O— or C₂H₅O—; and a is an integer of 0 to
 3. 5. A magnetic recording medium according to claim 4, wherein said alkoxysilane compound is methyl triethoxysilane, dimethyl diethoxysilane, phenyl triethoxysilane, diphenyl diethoxysilane, methyl trimethoxysilane, dimethyl dimethoxysilane, phenyl trimethoxysilane, diphenyl dimethoxysilane, isobutyl trimethoxysilane or decyl trimethoxysilane.
 6. A magnetic recording medium according to claim 1, wherein said polysiloxanes are represented by the general formula (II):

wherein R² is H— or CH₃—, and d is an integer of 15 to
 450. 7. A magnetic recording medium according to claim 6, wherein said polysiloxanes are compounds having methyl hydrogen siloxane units.
 8. A magnetic recording medium according to claim 3, wherein said polysiloxanes modified with at least one compound selected from the group consisting of polyethers, polyesters and epoxy compounds are represented by the general formula (III), (IV) or (V):

wherein R³ is —(—CH₂—)_(h)—; R⁴ is —(—CH₂—)_(i)—CH₃; R⁵ is —OH, —COOH, —CH═CH₂, —C(CH₃)═CH₂ or —(—CH₂—)_(j)—CH₃; R⁶ is —(—CH₂—)_(k)—CH₃; g and h are an integer of 1 to 15; i, j and k are an integer of 0 to 15; e is an integer of 1 to 50; and f is an integer of 1 to 300;

wherein R⁷, R⁸ and R⁹ are —(—CH₂—)_(q)— and may be the same or different; R¹⁰ is —OH, —COOH, —CH═CH₂, —C(CH₃)═CH₂ or —(—CH₂—)_(r)—CH₃; R¹¹ is —(—CH₂—)_(s)—CH₃; n and q are an integer of 1 to 15; r and s are an integer of 0 to 15; e′ is an integer of 1 to 50; and f′ is an integer of 1 to 300; or

wherein R¹² is —(—CH₂—)_(v)—; v is an integer of 1 to 15; t is an integer of 1 to 50; and u is an integer of 1 to
 300. 9. A magnetic recording medium according to claim 3, wherein said polysiloxanes whose molecular terminal is modified with at least one group selected from the group consisting of carboxylic acid groups, alcohol groups and a hydroxyl group are represented by the general formula (VI):

wherein R¹³ and R¹⁴ are —OH, R¹⁶OH or R¹⁷COOH and may be the same or different; R¹⁵ is —CH₃ or —C₆H₅; R¹⁶ and R¹⁷ are —(—CH₂—)_(y)—; y is an integer of 1 to 15; w is an integer of 1 to 200; and x is an integer of 0 to
 100. 10. A magnetic recording medium according to claim 1, wherein the amount of said coating organosilicon compounds is 0.02 to 5.0% by weight, calculated as Si, based on the total weight of the organosilicon compounds and said magnetic acicular particles.
 11. A magnetic recording medium according to claim 1, wherein said carbon black coat is obtained by mixing carbon black fine particles having a particle size of 0.005 to 0.05 μm with the magnetic acicular particles coated with at least one organosilicon compound while applying a shear force.
 12. A magnetic recording medium according to claim 1, wherein the thickness of said carbon black coat is not more than 0.06 μm.
 13. A magnetic recording medium according to claim 1, wherein said black magnetic acicular composite particles have an absorption amount of myristic acid of 0.01 to 0.3 mg/m².
 14. A magnetic recording medium according to claim 1, wherein said black magnetic acicular composite particles have an aspect ratio (average major axis diameter/average minor axis diameter) of 2.0:1 to 20.0:1.
 15. A magnetic recording medium according to claim 1, wherein said black magnetic acicular composite particles have a BET specific surface area value of 21 to 160 m²/g.
 16. A magnetic recording medium according to claim 1, wherein said black magnetic acicular composite particles have a blackness (L* value) of 15 to
 22. 17. A magnetic recording medium according to claim 1, wherein said black magnetic acicular composite particles have a volume resistivity of not more than 1.0×10⁶ Ω·cm.
 18. A magnetic recording medium according to claim 1, wherein said black magnetic acicular composite particles have a geometrical standard deviation of major axis diameter of 1.01 to 2.0.
 19. A magnetic recording medium according to claim 1, which further comprises a gloss of coating film of 165 to 300%, a surface roughness Ra of coating film of not more than 11.5 nm, a linear absorption of coating film of 1.75 to 10.0 μm⁻¹, and a surface electrical resistivity of not more than 10×10⁸ Ω/cm².
 20. A magnetic recording medium according to claim 1 or 2, which further comprises a non-magnetic undercoat layer disposed between said non-magnetic base film and said magnetic recording layer.
 21. A magnetic recording medium according to claim 20, which further comprises a gloss of coating film of 165 to 300%, a surface roughness Ra of coating film of not more than 11.0 nm, a linear absorption of coating film of 1.80 to 10.0 μm⁻¹, and a surface electrical resistivity of not more than 1.0×10⁸ Ω/cm².
 22. In a method of forming a magnetic recording medium comprising a non-magnetic base film, and a magnetic recording layer comprising a binder resin and magnetic particles, the improvement comprising using as magnetic particles black magnetic acicular composite particles having an average particle diameter of 0.051 to 0.35 μm, comprising magnetic acicular cobalt-coated iron oxide particles or magnetic acicular metal particles containing iron as a main component, a coating formed on surface of said magnetic acicular particles, comprising at least one organosilicon compound selected from the group consisting of: (1) organosilane compounds obtainable from alkoxysilane compounds, and (2) polysiloxanes or modified polysiloxanes, and a carbon black coat formed on said coating layer comprising said organosilicon compound, in an amount of from more than 10 to 40 parts by weight based on 100 parts by weight of said magnetic acicular particles.
 23. The method according to claim 22, wherein said magnetic acicular particles are particles having a coat which is formed on at least a part of the surface of said magnetic acicular particles and which comprises at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of 0.01 to 20% by weight, calculated as Al or SiO₂, based on the total weight of the magnetic acicular particles coated.
 24. A magnetic recording medium comprising: a non-magnetic base film; a non-magnetic undercoat layer formed on said non-magnetic base film; and a magnetic recording layer comprising a binder resin and black magnetic acicular composite particles having an average particle diameter of 0.051 to 0.35 μm, comprising magnetic acicular cobalt-coated iron oxide particles or magnetic acicular metal particles containing iron as a main component, a coating formed on surface of said magnetic acicular particles, comprising at least one organosilicon compound selected from the group consisting of: (1) organosilane compounds obtainable from alkoxysilane compounds, and (2) polysiloxanes or modified polysiloxanes, and a carbon black coat formed on said coating layer comprising said organosilicon compound, in an amount of 0.5 to 10 parts by weight based on 100 parts by weight of said magnetic acicular particles.
 25. A magnetic recording medium according to claim 24, wherein said magnetic acicular particles are particles having a coat which is formed on at least a part of the surface of said magnetic acicular particles and which comprises at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of 0.01 to 20% by weight, calculated as Al or SiO₂, based on the total weight of the magnetic acicular particles coated.
 26. A magnetic recording medium according to claim 24, wherein said modified polysiloxanes are compounds selected from the group consisting of: (A) polysiloxanes modified with at least one compound selected from the group consisting of polyethers, polyesters and epoxy compounds, and (B) polysiloxanes whose molecular terminal is modified with at least one group selected from the group consisting of carboxylic acid groups, alcohol groups and a hydroxyl group.
 27. A magnetic recording medium according to claim 24, wherein said alkoxysilane compound is represented by the general formula (I): R¹ _(a)SiX_(4−a)  (I) wherein R¹ is C₆H₅—, (CH₃)₂CHCH₂— or n-C_(b)H_(2b+1)− (wherein b is an integer of 1 to 18); X is CH₃O— or C₂H₅O—; and a is an integer of 0 to
 3. 